It's a bit of a rubbish situation really. You intend on spending your money on a ticket and would like to know exactly that ticket entitles you to. Staff generally can't/won't help to give you a list of routes you may take. If you endeavour to check yourself and use the National Routeing Guide, your findings are subject to a check of National Rail Enquires, which is not set up to verify the validity of tickets!
For example, if I want to find out for a customer, or myself, which routes can be taken on a Canterbury West to Ockendon routed "Tilbury&Gravesnd," it's not possible. Southeastern got Atos to rig the journey planner in August last year to ban any routes passing through Northfleet. A similar rig was put in place to ban routes passing through the Stratford Group. These days, neither RJIS nor NRE can produce any routes at all for the ticket!
I think it's one of those situations which is being left as it is until such a time that either a solution becomes available, or it becomes a significant problem. In the meantime, if you want to find out what the ticket you have purchased entitles you to, it's not going to happen!