Many quantities in After Effects are either points in time or spans of time, including the current time, layer In and Out points, and durations of layers, footage items, and compositions.
By default, After Effects displays time in Society of Motion Picture and Television Engineers (SMPTE) timecode: hours, minutes, seconds, and frames. You can change to another system of time display, such as frames, or feet and frames of 16mm or 35mm film.
You may want to see time values in Feet + Frames format, for example, if you are preparing a movie for eventual output to film; or in simple frame numbers if you plan to use your movie in an animation program such as Flash. The format you choose applies to the current project only.