Yes it is just for holding HP. It works a bit complicated, since when you stake hive to HP, you basicly convert hive to another token VEST in the background, and then this ratio vest to hive keeps growing in the favor of vests, and when you power down, aka convert back vests to hive it gives you more hive.
But the simple version is yes it is just for holding HP, and it is around 3%. This number also is variable and depends how much of the hive supply is staked.
The interest for HBD is a totaly diferent source of inflation.