Tail compatibility is the simplest compatibility feature to use, and it's a good example of how Mario's compatibility features work in general.
Normally when a character steals a power-up from Mario, they're healed for 12% damage. The Tanooki Leaf is slightly different in that the amount it heals by depends on whether the character has a tail. If you have a tail, you heal for 16%; if you don't, you heal for 10%. If your character has a tail, add this to your neo_data_compat:
Teammates receive quadruple health if they grab Mario's power-up, to encourage sharing. As a result, teammates who don't have tails will only receive 40% from a Tanooki Leaf, while teammates with tails will receive 64%! In contrast, a normal power-up will heal for 48%. If you make sure that your character has accurate tail compat, that can help Qua Mario players and their teammates make more tactical decisions about sharing their power-ups!

You can change this value later at any time by running neo_data_compat.has_tail = true; or = false;. This might be useful if your character changes form and sometimes doesn't have a tail. If your character never has a tail, then there's no need to include this at all - it defaults to assuming they don't.