Refers to products or services not in the Work, usually because the client is retaining it, or another contractor is responsible for it.
Useful primarily for identifying products and services that you might reasonably have assumed would be part of this Work, based on industry norms. Knowing you don’t have to do it allows you to reduce your price; conversely, not realizing something is out of scope drives your price up, making you less competitive.
See scope of work and scope creep.