What do the Tau battlesuit numbers 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.

Lexicanum references Codex:Tau Empire, 4th Edition, p.35, and says:


  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.