Here the definitions for System and Freeform games from our Guide
Games with no solid rules - they are played out entirely with the imagination. Freeform games are sometimes referred to as co-written stories, rather than as 'games', as they involve no element of random chance or mechanical unpredictability, both of which are usually found in system games.System Games
Games using rule systems including, but not limited to, Exalted, D&D, GURPS, and other custom rule sets. System games often make use of a dice-rolling system or character sheets.