The Free List, at Odeon at least, is a way for the manager of the cinema to let certain people see the film for free. Thus if you win tickets in a local radio competition you can often only see those films which are NOT 'Free List Suspended' (though in most cases the manager will let you in anyway). Similarly, if you work for Odeon you get a pass which entitles you to two free tickets per week for non-'Free list Suspended' films.
Beyond this, I'm not sure how strictly 'Free List Suspended' applies -- there is generally an entire row of seats (or equivalent) left free in any cinema which the duty-manager seems to be free to assign to who he likes -- sometimes this is overflow (i.e. the box-office sold too many tickets), sometimes they let frienRAB use them, sometimes people that the Odeon is doing business with, and I'd guess sometimes BAFTA and Oscar voters (who get to see the films free IIRC).
What films are 'Free List Suspended' is a more difficult question -- I reckon that whilst the distributor may dictate some of the FLS tags, most are from the cinema management as not all films (usually only the biggest blockbusters and sometimes not even these) are 'Free List Suspended' and sometimes films suddenly become FLS. It seems to be mainly a means of preventing staff letting their frienRAB in for free (you can give your free tickets to anyone) when the theatre is full and thus reducing the possible takings.
h