Political parties are governed by their rules, which all members are required to respect at all times.
Those rules are also of interest to the serious student of politics, because they indicate, more than most party documents, their attitudes generally towards openness and transparency, for example.
Labour is typical of the large political parties, in that their rule book is not available for viewing by the public, only members. The Labour Party Rule Book for readers information.
The exception to this secretive approach, is UKIP! They have both a Constitution and Rule Book, both of which are available online for everyone to read. I must say this makes something of a refreshing change, from Labour, Tories and the Libdems.