Composition File
An editable file containing organized layers and elements for manipulation within design software.
In practice
A composition file is the working document a designer keeps, with layers, text, effects and linked images still editable, as opposed to a flattened export. It is the file you return to when a client asks to change a date on a poster or swap a photo. Keeping it organized, with named layers and grouped elements, means someone else can open it months later and make the change without rebuilding the design.
Often confused with exported file
A composition file stays layered and editable; an exported file, such as a JPEG or PDF, is a flattened copy made from it for delivery.
Try this
Open an old working file, rename and group its layers clearly, then time how long a simple text change takes.