Half move -- "you move your AB"
Full move -- "you move twice your AB"
Charge -- "you move three times your AB, and a tack"
Run -- "you move 6 times your AB, it there is a -20/+20 to hit you"
There is no "up to your AB," just "your AB." There is no Charge without an attack. Does this mean if I mave a movement rate of 3/6/9/18, I cannot move 9 meters without making an attack? If I can, does it count as a Run, and do I get the -20/+20 effect, or not? If not, isn't it just a Full Move that has to be made in a straight line for the last 4 meters? Do I have to move my full 18 to get the -20/+20? Can Mega Nobs only move 12 meters if they attack? Otherwise they're magically crippled?
This is atrociously explained.
It makes sense to me.
Half a move: you move your AB
Example: Bob the weapons specialist has an agility score of 32. Therefore his agility bonus is 3. Bob decides he wants to move to the cover 3 meters in front of him (3 squares if your using a tactical map). Therefor Bob utilizes his half move and moves 3 squares, using up one half action. This leaves him a half an action to take a shot at the cultist who is shooting at his friend.
Full Move: Move Twice your AB. Using Bob again, with his AB of 3, this means 6 meters.
Example: Bob the weapons specialist realizes that the Ork grunt has flanked his current position and wants to fall back to the cover 6 meters (squares) to his rear. Thus Bob uses his full move (or two half actions), keeping his head down, to move to this new position.
Charge: You move 3x your AB and attack. So Bob with an AB of 3 can move 9 meters (squares) then attack with his Melee weapon.
Example: Bob the weapon specialist uses the last of his charge for his lascarbine. Bob sees that a secessionist soldier is engaged in a Melee with his squad ate about 9 meters away. Bob uses the charge action to move 9 meters and attempts to stab his enemy with the bayonet attached to his lascarbine. He'd get a couple bonuses for doing this, +10 for the charge and +10 because his target is already engaged with his squadmate.
Run: Move 6 times your AB. Bob with an AB of 3 can run 18 meters (squares).
Example: Realizng the horror of what is before him, Bob wants to get as far away from the force of Chaos Marrines approaching his position. Utilizing his full turn to run as fast as possible, bob sprints 18 meters in the other direction.
So what's the problem? Do you really need to be "told" that "move x times your AB" actually means "up to?" Just using your common sense should tell you this. If it wasn't the case you'd literally have a game where it was impossible to move a single meter / you'd only be able to move multiples of your AB. So, to answer your question: if you have Bob and he wants to move 9 meters, but is not purposefully charging a target, he'd have to "run," giving him a total available move of 18, of which he'd use 9.
Why does he have to get the "run" bonus and penalty when he does this? Keep in mind that these rules are highly abstracted. For example, a full move of "6" for Bob represents Bob moving purposefully from point x to point z--just fast enough to keep his head down and to stay totally "combat aware" of his surroundings. A charge (which can be any distance from 3 to 9) represents Bob screaming and running headlong at an enemy with the intent to stab/chop him in the face. This is causes a totally different set of reactions for his enemies than him simply "running" 9 meters.
Which leaves running. For Bob to move 7 or more meters, and not be running straight at the enemy with bayonet down, requires him to move faster and thus be less in control of his movements and less combat aware, thus inducing certain penalties and giving certain bonuses (e.g it's harder for his enemies to lead him).
Really it is similar in almost every other game to some extent. For example in dnd 4e you can "charge" and get +2 move and a +2 to attack. So a character with the speed (move) of 6 gets that bumped up to 8 AND he gets a bonus. Now say the same character with a speed of 6 wants to move 8, but isn't charging at an enemy… Now he has to "run" instead, which means he gets to move, but now he grants combat advantage (his enemies and not him will get the +2 attack) AND he takes a -5 to any attacks he makes until his next turn starts