Function sync_part 

fn sync_part(
    client: &Client,
    account: &str,
    listed: &Listed,
    report: &mut Report,
    first: bool,
    until: Option<Instant>,
) -> Result<bool, SyncError>

One collection’s turn (sync_with): on its first (first), what changed here is sent; then what changed there is brought until until, batch by batch. Whether it is whole now: else the next turn goes on from there.