Also called: block-level incremental, block delta backup
In plain English
A block incremental backup goes finer than whole files: within a changed file it stores only the individual blocks (pages) that changed, rather than re-copying the entire file. pgBackRest introduced this in version 2.46. It compares block checksums against the previous backup to decide what to store.
Why it matters
For a large, mostly-static table — say 500 GB that changes a few pages a day — file-level incrementals would still re-copy whole changed files, but block incrementals capture just the handful of changed pages, shrinking backup size and time dramatically.