The Tau Stealth suit is a little difficult to understand. Mainly because FFG does not go into significant detail concerning its skills or how they work in the setting. I am specifically discussing concealment in this post. So I will go over the mechanics of concealment and how they work with the Tau Stealth Suit.
First off: lets go over Concealment.
The concealment skill is a basic skill based upon agility. This means you can attempt it at half characteristic skill even if untrained.
It is always an opposed test. If no-one is around there is no need for a test (GMs may make you take one anyway to make you think there is someone around though). Its kind of pointless to try to hide from no-one. Concealment requires an active concealment action (half action) taken each turn to use concealment. If you take an action which requires more than a half action, or fail to use concealment in a turn, your concealment drops. There are also many things which can affect the difficulty of the concealment test the GM needs to always take into consideration: the sneaky git's actions, the observer's direction of view, the observer's state of alertness, distance between sneaky git and observer, and local effects when determining any skill modifiers to the opposed test.
--Sneaky git's actions mean: How fast is the sneaker moving? Generally the faster a person moves the more he/she stands out against the background (standing still gives you +10 and you generally can't move faster than a half-move when concealed). If detected previously and re-concealing himself but not moving from his last position would make it easier to be found again as well. Also, has the sneaker done anything that would reveal his/her presence? Shooting a weapon or making an attack will end concealment although this may not always make the sneaker seen (a long range shot with a stalker boltgun and stalker ammo would end concealment but since enemies cannot detect the shot the character would generally have enough time to re-conceal himself before being detected normally).
--The observer's direction of view means just that, which way the observer is looking. If the observer isn't looking at you a concealment test isn't generally needed as the observer shouldn't be able to see you (silent move on the other hand is a different story). The sneaking character should still be using concealment though, as others may be looking his way or the observer may turn around before the character gets a chance to activate concealment in his next turn.
--The observer's state of alertness means how intent or perceptive the observer is at the moment. A sleepy guard is much less likely to see someone sneaking in the bushes than a guard on full alert after hearing gunshots.
--Distance between the sneaker and observer is pretty self-explanitory. The farther away something is the harder it is to see. If you are out at a range a character would have to make an awareness test to see you in the open, your concealment test should be much easier than normal.
--Local Effects means whats going on around the sneaker. Is there a sandstorm, heavy winds, or dead calm? Is the sneaker walking through a stream, a puddle of mud, or hard rock? Its harder to stay concealed if there is a large number of muddy foot impressions pointing right to where you are standing. Sand storms might be good for normal sneaking but would probably cause havok on active stealth systems though.
Those are all the efefcts a GM should always take into account, but it shouldn't be too difficult to assign these values on a spur of the moment basis. It just recquires an understanding of the skill itself.
Once the values are set the sneaker and the observer must roll concealment (as long as the character has spent his action every turn to remain concealed) verses awareness. If the sneaker wins the opposed test or passes his test and the observer fails his awareness roll the sneaker remains unseen. If the sneaker looses the opposed test or fails his concealment roll the observer sees him without any penalty (normally, stealth suits have the further penalty will get to that in a moment). Of course, your GM milage may vary, you can use as much or as little of the above in your games as you want, but just remember if your players or characters try to get into position to improve their skill chances or do something stupid which degrades their skill chances you should modify the skill difficulty accordingly.
Whew that was a bit wasn't it. Well these skills are a bit more complicated than just rollx + y. Not to knock FFG but this is why many games have a "player book" and a "GM book". Where the GM book goes into the in-depth description of how the different rules work and how the GM should use them in games.
Now that we've gone over concealment lets talk about stealth suits. They are pretty snazzy eh? But despite all their fancy pants rules, they don't ignore the rules for concealment.
Stealth suits have two modes to their stealth generators which they may switch between as a free action each turn. I will go over both modes and how their rules affect concealment.
The first is passive mode. This mode gives a -10 penalty to awareness, BS and WS tests. The -10 to awareness is only used when the stealth suit is using concealment and/or specific situations which require awareness tests (such as trying to be seen over long distances), although the penalty to BS and WS are probably "on" all the time. This penalty only applies to sight though. Creatures who use hearing or smell (Unnatural senses for instance) as well as silent move checks are NOT affected in passive mode (auto-senses may ignore this as well, I will add this to a question submission to FFG). Those creatures who "see" with hearing, smell, or other unnatural senses would not get a -10 penalty to awareness, BS, or WS tests either, nor would a character using the Blind Fighting talent.
The suit's active mode is much more complex. I figure it includes some sound canceling technology in addition to active cloaking tech (although how it dampens smell is beyond me). As such all senses (including hearing and smell, I think) are affected by a -30 penalty to awareness, BS, and WS tests when active mode is on. Again, the -30 to awareness is only if the suit is currently using the concealment skill and/or specific situations which require awareness tests (such as trying to be seen over long distances), although the BS and WS penalties are likely all the time.
The suit actively jams sensors and the like which can be detected much easier than the suit itself. If a suit is in active mode a character who attempts an awareness test to detect a stealth suit will detect the jamming if he gets 1 or more Degrees of Success (DoS) on his awareness test (the character does not need to win the opposed concealment test to detect the jamming).
Example: a space marine with auto-senses and perception 40 (awareness skill of 60) vs. a stealth suit in active mode; concealment skill of 58:
The stealth suit rolls a 32 for concealment and the space marine rolls a 20 for awareness. The space marine was testing on a 30 due to the stealth suit's active field and gets only 1 DoS. The stealth suit was testing off of 58 and gets 2 DoS. The stealth suit remains concealed from the space marine, but since the space marine got 1 DoS on his awareness test he detects the jamming of the stealth suits active system on his auto-senses. If this space marine has faced stealth suits before and seen their jammin effects it is fairly easy to believe he could identify the jamming source as a nearby stealth suit.
The suit still follows normal rules for concealment, in the above example, had the suit failed its concealment roll the space marine would have detected the suit and had the space marine failed his awareness roll he wouldn't have been able to detect the jamming. Also, once the suit pilot fires his burst cannon or fails to use concealment he then drops from concealment and can be seen. Although it is possible for a stealth suit to fire his burst cannon (half-action with auto-stabilised) then re-conceal himself with his other half action. Also, while a stealth suit has bonuses to concealment it has no bonuses whatsover to silent move, they don't even have silent move trained. It should be much easier to detect the sound of a moving stealth suit than see it, as the pilot is more apt to fail a silent move test on a test on 19 than a concealment test on a 58 (GMs, just be sure to keep them out of earshot when using them).
FInally, there are a few rules that may affect a stealth suit and its concealment options. Notably its size (not listed in the rulebook) and its armor rating (armor with AP greater than 7 gives -30 to concealment and silent move tests. If the suit is hulking there is no helping it, the suits will get a -10 to concealment. Though I doubt they are hulking. I'm fairly sure the FFG guys intended the stealth suit to be an exception to the AP 7 rule but for both of these I will send off a question to the rules guys.
Hope that helps. I know it was a lot but as I said before, FFG doesn't go into enough detail with their skills which leaves them a little ambiguous at times.