This is something that I've built up over a few years, and not without considerable reference of this forum, and what others have done in the same field.
First thing first: This system doesn't use octal. I don't use octal, so my system doesn't. It uses decimal. I truly don't care if Tau do use octal. I'm more than happy to say that this is an "Imperial" system assigned to xenos vehicles.
As the Tau Empire grows and more, newer battlesuits are developed, a designation like "XV-22" can start to get muddied. Presumably there is some extended, internal reference code that the military within the Tau Empire would know, but, much like saying "an Abrams tank", XV-22 fits for ordinary usage.
But that still isn't quite specific enough, and I decided that Tau battlesuits should be recognized by three things:
b) mass class
The addition of generation provides additional information (broadly) about the battlesuit's conception and purpose, while the other identifiers already exist in the current system, but still see modifications in this system.
The first number is effectively limitless, but will never practically reach very high. The current cutting-edge, Shadowsun's XV-22 and Riptides, are only generation 3 under this system. The Riptide family could be considered generation 4, but as the XV-22 had not yet gone into production when they first appeared, I'm classing them as the same generation.
With the inclusion of the T- and S-series, it can be bumped up further, though this is largely unnecessary as they are irrelevant to both the game, and modern Tau warfare. Propeller-driven planes are not included in the generational numbering of jet fighters.
If the S- and T-series are ignored, then the XV-8 is generation 1. As it was the "original" battlesuit, the 1 from its designation can be dropped. So too could other first-gen battlesuits, but this is a special concession to keeping "XV-8" unchanged.
This remains mostly unchanged. The use of decimal in place of octal does notably add a "true" size 9 and 10.
0 can be considered Fire Warrior armour, although it is not a true battlesuit. The rest of the numbers remain the same, with 8 being the "standard". With the addition of 9 and 10, both new mass classes would be exceptionally large, the Riptide family occupying the 10 slot.
Role is still a difficult subject, and several positions in this system I feel are poorly assigned and/or described. (Those being 3, 4, 6, and 7.) We simply don't have a complete picture of every number, and in many instances only have a single, specialised battlesuit using a certain number. This allows a certain amount of information to be gleaned and inferred, but it can never be called truly correct or "canon," unfortunately. Again, the use of decimal adds two more "true" role numbers.
The duties/types assigned to each role number are based primarily on ATT's own system, as well as the work others have done on it in their own systems. I feel certain roles (like electronic warfare and C&C) are not necessarily something a battlesuit would be doing, and I am not entirely convinced of others, even within my own system. (Space, especially.)
Some roles are pure conjecture and guesswork, and definitely need to be changed with more insight and information.
Role numbering also includes a 0, which can be dropped from common usage, and is again a concession to the XV-8.
0 - multi-purpose
1 - fire support
2 - experimental/prototype
3 - electronic warfare/networking
4 - C&C
5 - stealth
6 - space
7 - engineer/battlefield support
8 - heavy weapons
9 - heavy armour/VIP
10 - line-holder
The first column lists battlesuits as we know them, as well as some theoretical examples. The second column gives their designation in this alternative system. The final column explains the second column.
XV-8 = XV-180 = 1st-gen, size 8, multi-purpose
XV-15 = XV-115 = 1st-gen, size 1, stealth
XV-25 = XV-225 = 2nd-gen, size 2, stealth
XV-22 = XV-322 = 3rd-gen, size 2, experimental
XV-88 = XV-188 = 1st-gen, size 8, heavy weapons
XV-46 = XV-246 = 2nd-gen, size 4, space operations
XV-104 = XV-3108 = 3rd-gen, size 10, heavy weapons
XV-107 = XV-3101 = 3rd-gen, size 10, fire support
XV-82 = XV-382 = 3rd-gen, size 8, experimental
XV-?? = XV-31010 = 3rd-gen, size 10, lineholder
Including the S- and T-series results in the following generational changes:
T-8 = T-180 = 1st-gen, size 8, multirole
S-8 = S-280 = 2nd-gen, size 8, multirole
XV-8 = XV-380 = 3rd-gen, size 8, multirole
XV-25 = XV-425 = 4th-gen, size 2, stealth
XV-22 = XV-522 = 5th-gen, size 2, experimental
XV-82 = XV-582 = 5th-gen, size 8, experimental
This was not meant to be presented as THE way. It is simple another way of considering what "XV-8" means. I am not saying that my way is right, and ATT's is wrong. But, I'm also not saying ATT's is right and mine is wrong. This is just another alternate take on it, and I hope it provided a fresh look at Tau battlesuit designation and numbering.
Thank you reading, and letting me share this.