Mark of the Xenos contains a little information on the naming and numbering of Tau Battlesuits, expanding on a notion mentioned in Andy Hoare's novel Savage Scars and a concept mentioned in the Tau Empire codex (if memory serves).
Essentially, the term XV derives from a corruption of the Tau word "Hereks'vre" (or "Her'ex'vre"; Savage Scars uses the former spelling, MotX uses the latter) which means "mantle of the hero", adopted by Imperial xenologists during the Damocles Crusade. The numbers following the XV designation refer first to the suit's "mass class", with 1 referring to ultralight suits (barely larger than a normal infantryman), 2 referring to light suits (about Astartes-size), and 8 referring to the heavy suits (about the size of a Tyranid warrior, a little smaller than a Sentinel walker), and then to a specified role - no second digit means utility/variable-role, and there's a full list in MotX, on page 12, summarised here:
- Theoretical/developmental models
- prototypes in field trials
- suits not in mass production with prototypes permanently assigned to a pilot
- command and control
- infiltration units
- fire support
Now, obviously, given that we only actually know of a handful of types of Battlesuit (and acknowledging that the numbering doesn't quite fit with the models produced by Forge World - by the MotX numbering, the XV81 would be an experimental/theoretical heavy battlesuit, while the XV89 is easier to deal with as 9 isn't defined in MotX, so we can assume it refers to a heavily-armoured suit, and the newer XV9 suits clearly represent the superheavy mass class, being bigger than the mass-class 8 Crisis Suits), this numbering system leaves a lot of room for other models of Battlesuit unseen in the wargame... given the Tau propensity towards technological development, this provides us with the opportunity to consider what those other suits might be like, potentially even to use them in game.
I've got a few ideas buzzing around (XV18 ultralight fire support battlesuits - tiny suits, but really agile with missile launchers for rapid-response fire support), but I thought I'd get the basic premise posted before I cook up anything more specific, see if anyone else can come up with something.