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.
The Warhammer 40K Wiki has more detail:
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.