It we create a new equipment, like a dagger or a spear, we can't add any new custom enchantment to them without making them available to vanilla tools too.
For example, if we wanted to make some dagger-exclusive enchantments (like poisonous or backstab), they would have to be classified as sword enchantments, but they would no longer be exclusive to daggers then.
It would be cool if there was a "custom" enchantment group and then we would be able to manually add the available enchantments to a tool while editing its tab (or maybe it would be better if it was actually possible to create a new enchantment group for each new tool, but I don't know how vanilla code works).
This is a bit out of scope at the moment, unfortunately, but I will keep this in mind for the future ;)
You can limit your enchantment to a set of items already, but I understand this does not cover tabs and such yet.