Souls

Normal SOULs
What you will probably be using while playing the game up to reset 7+ (unless you have an event soul). Any soul that can be obtained by exchanging SOUL fragments and gold at the soul shop typically fall under this category.

True SOULs
Upgraded forms of the previous souls and what you'll mainly be using for getting true resets. These usually require 5M Gold, the original SOUL, and 4 fragments, and are permanent; The only exception to this is True Determination, requiring 10M Gold, the original SOUL, and 2 True Determination fragments.

Boss/Event SOULs
Souls that are obtained right after defeating a normal/event boss, and are usually permanent. Also what you'll most likely be using instead of basic SOULs.

Exceptions
Reserved for SOULs that don't fit in the above