It very much depends who you work for.
In most instances, it is free travel on your TOC for residential, duty and leisure purposes for you as an employee.
Your spouse/partner and dependants will receive free travel for leisure purposes only (children also receive free travel to places of education in most instances).
In terms of free travel on other tocs under the same parent company - it is very much dependant on what company you work for. First Group as an example do not give free residential travel to staff employed on sister TOCs and you do not recieve unlimited use for leisure travel.
Abellio and Arriva on the other hand do allow unlimited leisure and residential travel on sister TOCs. It will all be outlined and made abundantly clear to any new starters what they can and cannot do with their travel facilties.
Misuse is treated very seriously by most employers.