An Open Setup is one wherein the exact role distribution in the game is known.
This type of game tends to be relatively simple, and often these setups are designed with a particular purpose in mind.
A huge advantage to Open setups is that they can be replayed without any loss of enjoyment, as everyone knew from the start what the setup was going to be like. Contrast Closed Setups, which are usually only intended to be run once.
A disadvantage to Open setups is that they are highly subject to scrutiny. If it is possible to break them or otherwise find a dominant strategy, it will generally be found with reasonable speed.