Syncing, or pulling, is bringing your local copy up to date with the server. On a game project this is the operation people spend the most wall-clock time on, so how it behaves matters more than almost anything else about the system.
A naive sync compares timestamps or re-downloads whole directories, which on a 200 GB Unreal project means an artist loses their morning. An incremental sync compares content hashes to determine what genuinely differs and transfers only that, which turns the same operation into a minute or two.
The distinction becomes sharper in Unreal Engine 5. With One File Per Actor, a level edit might change forty actor files out of thirty thousand. A system that reasons about what changed moves forty files; a system that does not moves everything.
USourceControl syncs incrementally using SHA-256 hashes, verifies content on arrival, and resumes interrupted transfers rather than restarting them. Sync never overwrites local changes you have not committed, which is the property that makes it safe to run without thinking.