The free game is the full standard game: the regular board, the regular costs, ten points to win, the robber on a 7, the usual development deck, Longest Road and Largest Army. Three to six players, as many games as you want, on the same server a subscriber’s game runs on.
Nobody needs an account for it. That includes the host. Trading, chat, reconnecting to your seat and the replay at the end are all part of it.
What is not free is playing with any of it changed. Move one ore tile, add one of the 33 preset house rules or 10 preset awards, or put a clock on turns, and starting that game needs a subscription. The line is drawn per change, and each change is named in plain words — “Ore tiles: 4 instead of 3” — so you can see exactly what you would be paying for, or put it back. See the presets →
Re-rolling the board is not a change. A different deal of the same standard rules is still the standard game, and stays free.