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