Click on a client file in the Files menu. In the submenus that open, click on the Integrations submenu and then on the DATEV tile.
On this screen, you can set which Client ID (file in DATEV) corresponds to the current file in Billit.
You can then specify which document types the purchase and sales invoices can be assigned to.
Finally, you can set which document types correspond to the various documents and click Save.