Frame 1:An .fla file, the source or authoring file for Flash, appears to the programmer artist as layers of film strips. Each film strip is made up of frames to be delivered at a frame rate, which is set by the author.
From Reading Project
Layers are just one of the many metaphors Flash uses to communicate with its users and to convert the abstractions of its programming environment into more familiar terms. The interface simulates a drafting table or light box—concrete physical elements in the working environment of visual artists. The software enables users the option to drag cutout shapes around in space rather than having to specify those shapes and their movements mathematically. However, Flash is also built on spatial and temporal metaphors adapted from film and theater. For example, Flash employs the conceptual term “frame” (as in a frame in a film) to refer to a single segment of the digital animation; “timeline” describes the visual representation of a series of frames (similar to a film strip); “stage” designates the area that will appear on-screen when the program is run; “keyframes” are the beginning and ending points of the animation; and “Tween” describes the animated transitions and transformations that fill the frames in between two keyframes. When building an animation in Flash, the programmer views the authorware screen from a point-of-view positioned above “the stage” and looks downward onto its interface. This visual metaphor is a convention used in many popular image authoring programs (such as Photoshop or Gimp). The Flash manual compares its layers to “transparent sheets of acetate stacked on top of each other.” However, because Flash is time-based, the authoring environment also provides a cinematic timeline view that displays a stack of layers, each made of a strip of frames which extend the duration of the piece.
(Reading Project, 15-16)