"Realm" option at Add/Modify Price Plan page in Admin Area defines price plan visibility at "Select Price Plan" page in User Front-end when user places/renews his ad or selects/changes/extends his membership plan, see examples below.

For FEEs - Visibility at "Select Listing price plan" page:

Realm = Add Listing + Renew Listing
Always visible

Realm = Add  Listing
Show for new listings only (if listing's exp.date equals zero)

Realm = Renew Listing
Show for old listings only (if listing's exp.date > zero)

For SUBs - Visibility at "Select Membership price plan" page:

Realm = New Membership + Renew Membership
Always visible

Realm = New Membership
Show for new user accounts only (if user account exp.date equals zero)

Realm = Renew Membership
Show for old user accounts only (if user account exp.date > zero)