Presumably, if you have read this far, you already know what a screenplay is.
In the sfsg, we use the standard studio screenplay format — fixed-width fonts, one-minute-per-page. There is a lot of free and not-free software out there to help you create this format and whatever the format's shortcomings, it's way too late in the day to change.
Screenplays should always be submitted in PDF format. Microsoft Word is not an acceptable substitute.
All sfsg screenplays must be original material — no adaptations from other media and no sequels, prequels, or remakes. Even if it's in the public domain. Even if you own the rights.
Beyond that, we are very flexible. Length should be appropriate to the story and the audience. We don't give a fig about accuracy or produceability. If your screenplay could hypothetically reduced to sound and moving images — and it's good — we would like to read it.