A virtual source is a clip consisting of a single frame that behaves as if it were repeated indefinitely. When edited into the timeline, it can be trimmed, slipped, or slid to any extent—the virtual source automatically generates new frames on an as-needed basis. Head and tail frame counts are usually numeric, but because virtual sources have unlimited heads and tails, these are indicated by an asterisk (“*”).
You can create a virtual source in any of the following ways:
Alt-click the timeline element.
To view information about a virtual source clip, zoom in on the timeline or(a) Asterisk indicates unlimited head frames (b) Virtual source (c) Asterisk indicates unlimited tail frames