As of now, we don't have the list of variables that can be used when sending invoices from QuickBooks Online (QBO). However, these variables are always enclosed with the square brackets same as the example you've provided above.
You can also check them under the Account and Settings by following these steps:
Log in to your QBO account.
Click on the Gear icon.
Select Account and Settings.
Click on Sales at the left pane.
Go to the Messages section and check for the variables from there.
Click Save, then Done.
I'm adding these screenshots for additional reference.
You may also read this article for additional reference about sending invoices: Send an invoice.
Let me know if you have any other questions, I'm just here to help. Wishing you continued success!