They aren't documented, as far as I know.   I have been able to deduce some info, so use this carefully and at your own risk.  I looked at this over a year ago, so some of these might be a little different now.

REMOVE_ACCOUNT_PERMISSION - A result of a permission review where a permission assignment is revoked from an account. Specifies target account and target permission.

ADD_PERMISSION_TO_USER - After a request, assigns a new permisison to a account in a system.(WHICH ACCOUNT?)

REMOVE_ACCOUNT_ASSIGNMENT - During account review, if you remove the user from an account this event is sent to the application with the identity name and the account.

ADD_USER_TO_ACCOUNT - During a account review, if you enable modify permission you can assign unmapped accounts to a user/identity. This lists as “Assign User to Account” in fulfillment, and links the account object to the Identity. If you use a custom fulfiller action to select a user, from config, this does not work - its just a modify account type. Note that this is a change sent to the fulfiller (ad and edir do not support) and would likely entail an admin manually updating an account to have a matching attribute.

MODIFY_ACCOUNT - During an review you can enable custom modify actions via config..Modify Review Item Reasons. These changes become modify accounts.

ADD_APPLICATION_TO_USER- As a result of a request for an account, adds an account to a target application. Still needs collection for validation. The fulfiller needs to populate all appropriate attributes for creation/matching/placement. The default script uses a userProfile attribute that is configured in Config…Fulfillment Context Attributes

REMOVE_ACCOUNT- As a result of a account review, where an account is revoked. Specifies the target account in the target system that will be removed.

MODIFY_USER_PROFILE- After a user profile review, sends changes to a system (not supported by AD or eDir fulfillers?). Attributes must have the checkbox for available to review before showing up here.

MODIFY_USER_SUPERVISOR - After a user direct reports review, to confirm managers, this type changes the manager (not supported by AD or eDir fulfillers)