The meaning of this option varies depending on the action. If `All' is selected, then all the frames will be read from the file, or all the frames will be written to the file, or all the frames will be deleted from memory.
The other option is `Selected.' This will bring up three controls, labeled Begin, End, and Skip. These make it possible to use a subset of the frames, starting at frame Begin and selecting every Skip frames until the End is reached. For instance, to select every fifth frame between frames 14 and 98, set:
(Remember that frame numbers in VMD start at 0, so frame 0 is the first frame.) The value `-1' is a special number; setting Begin to -1 is the same as starting at the first frame, End = -1 is the same as ending at the last frame, and Skip = -1 is the same as taking one step.
When the Action is `Read File', the selection is applied to the frames from the file to be read. When it is `Write File', the selection determines the frames to be written, and when `Delete Frames', the selection determines the frames to be deleted from memory.