The rules that decide which snapshots to keep and which to let go over time.
A retention policy is the set of rules that decides which snapshots a backup set keeps and which it discards as they age.
The classic shape is tiered: keep every snapshot for the last 7 days, one per day for the last 30 days, one per week for the last year, one per month forever. This gives you fine-grained history for the recent past (you can roll back a bad edit to 2pm yesterday) and coarse history for the distant past (you can still find what your Documents folder looked like in June 2024) without paying to store every hourly snapshot from two years ago. A retention policy is a declaration of intent — rules expressed in days, weeks, months, counts — not an action.
The action that applies those rules is pruning, and it is a separate step. The retention policy says “this snapshot from 8 months ago is no longer needed by any rule.” Pruning actually removes the snapshot’s manifest and reclaims any unique data it held. Keeping the two concepts distinct matters: you can tune retention without running pruning, and pruning without a retention policy has nothing to work from.
In macup, every backup set has a retention policy you can see and edit in the app. The default is sensible for most people — hourly for a week, daily for a month, weekly for a year, monthly for five years — and you can tighten or loosen it per set without touching the snapshots that exist today.