Zones and Envelopes are used primarily in Simplified Reps.
Zones allow you to divide your model up into sections or areas. For example, one side or the other of a datum plane or a surface; a given distance from an entity like a vertex or an edge; or inside or outside an enclosed quilt. Once you have zones created, you can use them for selecting components to apply actions to in a simplified rep. For example, everything within the zone called MAIN, apply the geometry to.
Envelopes are used for substituting within a simplified rep-- replacing multiple components, even crossing subassembly boundaries, with a simpler part with much less geometry, so that you have a good visual reference that has less of a burden on your computer's memory than the original components. You can create an envelope part in the assembly by defining simple protrusions or cuts, or you can use the shrinkwrap functionality.
For more information, consult Pro E's built-in Help library.