As far as I can tell, 2da files that work server-side without the player having the same copy are ones that do not reference a model, sound, animation, or gui feature (such as leveling up).
But, there are circumstances where players don't need the updated version, such as when the change really only affects the server.
Example: spells.2da
Players need the updated version if you are adding a new spell or altering things like metamagic, spell level, target type, or spell description (.tlk reference number).
Players don't need it if you change something like the impact script that is read only by the server.
For your tail.2da, players do need that one, since it's all about which models to render and if it shows environmental reflection.