Tau battle suits are often dubbed XV88 or XV81, is there a pattern to the number? What does it mean?
The common consensus is that the Battlesuit number is XV (indicating Battlesuit), followed by two digits, constructed based on size/mass (1-9, light to heavy) and function (1=long-range?, 2=experimental, 4=high-precision(?), 5=stealth, 6=space-faring(?), 8=heavy support), although the functional categories are less clear.
Standard nomenclature has these battlesuits categorized by two numbers - the first number denoting which mass class it belongs to, the second its role. For example, in the XV22 Stealth Battlesuit, the first '2' denotes it as a lighter-weight suit, while the second '2' means it is still undergoing field-testing.
The Warhammer 40K Wiki has more detail:
All Tau Battlesuits are essentially the same pieces of Tau technology but can be customized and then given an alphabetical classification code of XV, which is followed by two digits. The first digit indicates the size of the suit: "1" being a complete exoskeletal version of standard Tau body armour, while "8" represents the largest available Battlesuits, the Crisis Battlesuits. The second digit indicates the role or status of the particular suit design. For example: "2" indicates an experimental design, "5" represents a Battlesuit with stealth capability, and "8" denotes a Battlesuit outfitted to provide heavy fire support. Other variations of a Battlesuit may be given different numbers to indicate their functions.
There is also the XV9 close combat suit, which has a slightly different naming scheme, and comes in two weapon flavours, the XV9-01 and XV9-04.
Additionally, the 5th edition codex mentions the XV16 and XV26. There is some speculation that the "6" may refer to a new functional designation, possibly space-faring.
Other letters represent different troop types. A detailed (exhaustive?) attempt to summarise them all can be found here.
It's a tip of the hat to real life military vehicle numbering which uses letters followed by a model number(F18, M1, ext.). The letter usually represents a type of item: T for tank, F for Fighter, X for experimental, and so on. The number is the sequence of the model. F-18 is the the 18th fighter model designation used by the US military.*
It may also be a possible reference to a few popular mecha animes. XV-88 is pretty close to RX-78 the designation of the mecha in the original Gundam series. This could be a coincidence, but the Tau do borrow some of their visual ascetic from Japanese manga and anime.
I don't think there's a literal meaning for XV other than it sounds cool and follows the pattern.
*This is after the US military reset their designations. They where getting up into the f-100s.