What is the purpose of the authorisation groups?

The authorization groups can be used to restrict functionalities and access. For example, a user in the Business account can only pay invoices if they have activated the "Pay" right.

The authorization groups are included from the Business Plus subscription. The groups can be created and edited in the Business account settings. Clicking on the name of the authorization group takes you to the settings where you can adjust the individual authorizations and add group members. 

The authorization groups are not related to the mail distribution groups. The distribution groups regulate to which employees a document is distributed, the authorization groups regulate which functions an employee can use.