Disposition rules gives the ability to customize the ACW experience on a skill level.  Rules can be set to contact (who), task (what), and notes fields.  The following rules are available:

REQUIRED - The field is required in order to submit the disposition.
EITHER_REQUIRED - Either Contact (who) OR Task (what) is required to submit the disposition.
NOT_REQUIRED - The field is not required to submit the disposition.
DISABLED - The field becomes no longer visible on the ACW screen.

A skill can only belong to 1 rule.

An override disposition is required for any rule containing a REQUIRED or EITHER_REQUIRED Task (what) or Contact (who).  This is because there needs to be a way out of the disposition screen if there is not an option for a Task (what) or Contact (who).  The override disposition will treat the Task (what) and Contact (who) fields as NOT_REQUIRED.