Definition
A scope register lists every deliverable, the number of revision rounds, the file formats, and the explicit exclusions. It is written before work starts and does not change silently.
The exclusions section is the important one
Anyone can list what is included. The exclusions prevent the argument three weeks in. Typical exclusions: copywriting, stock licensing, third-party plugin fees, hosting, print supervision.
Changing it
A change request is written, priced in time, and either accepted or declined. Accepted changes move the date; that is stated at the same time.
Reading it before you sign
Check three things: the file list, the round count, and the exclusions. If any of those surprise you, ask before booking rather than after.