This FAQ page is dedicated to the 1.1 version of Sandy only.

A screen is the class used to display the rendered objects, so every application must have a screen.

A clip screen draws each face into a dedicated clip. Every clip is deleted and then recreated every time the world is rendered!

A bitmap screen uses two bitmaps and a single clip to draw the whole scene. It uses double-buffering to avoid some visual problems. In consequence this class use the latest capabilities of Flash, and needs to export the animation in player 8.

I noticed a problem with _alpha transparency and the BitmapScreen, so it's possible to have some visual problems if you play a lot with the object's transparency.

About the performance, they are quite close, but Clipscreen seems faster.

ClipScreen use is recommended in most cases.