They are all part of Portsmouth Group.
Customers travelling between Chichester and Cosham may double back between Hilsea and Fratton. This easement applies in both directions.
Cosham is a member of Portsmouth Group so if your ticket is Cosham - Chichester then you're not permitted otherwise without the easement to double-back within Portsmouth Group as it is the origin.
So positive double-back easement. You're not allowed to double-back any further.