A named group of folders and rules that define what a single backup job protects.
A backup set is a named collection of source paths, exclusion rules, and a destination — the unit a backup engine runs against to produce snapshots.
Think of it as the answer to three questions bundled together. What folders should be protected? What should be skipped inside them (caches, node_modules, the Downloads folder, a specific scratch disk)? And where should the resulting snapshots be written? A Mac might have one backup set called “Documents and Photos” that points at the home folder minus a few caches and writes to macup Cloud, and a second set called “Video projects” that points at an external SSD full of Final Cut libraries and writes to a BYO-storage bucket. Each set is scheduled and runs independently.
Keeping sets separate is how you avoid awkward tradeoffs. Your small-but-precious writing folder does not want the same retention policy as a 4TB video scratch disk, and mixing them in one set forces a compromise. Splitting by intent — what is this data and how long should it live — lets each set get rules that fit.
In macup, every protected location shows up as a backup set in the app, with its own schedule, its own destination, and its own timeline. When a set runs, it produces one snapshot, tagged with the Mac it came from, which is what you browse when you open restore.