There are three different types of syncs that happen between Re-Leased and QuickBooks Online.
Automatic sync happens every 10 minutes.
This will synchronise all data between QuickBooks Online and Re-Leased.
If an Invoice or Credit Note is edited/created in Re-Leased, a sync will happen for the individual Invoice/Credit Note.
Lastly, a manual sync can be carried out via Settings > Manage Add-ons and integrations > QuickBooks Online add on. You can force all data to be synchronised using the sync all data button.