Role of Persistent Level in World Composition

Please explain in detail the role/job/function of the persistent level (PL) when using world composition.

Subquestion 1: What is placed inside the PL in regards to level design and overall game functionallity?

Subqeustion 2: Do Actors which need to be persistent over all existing levels (regardless of their local loaded/unloaded state) need to be added to the PL ?