The eligiblity based National railcards [senior, young persons (& older students), disabled, family, ....] all have slightly different terms & conditions and restrictions.

However in so far as the Network Railcard, which is valid in the south east area only, is concerend while it is not such restricted and open to all, do note that there is a minimum fare Mondays to Fridays.

However there is no minimum fare at weekends.