Usually, customers are sent an invoice immediately after payment, allowing them to process it in their administration. When customers enable monthly billing, they no longer receive an invoice immediately after payment. With monthly billing, customers receive an invoice every 1st day of the month, containing an overview of all payments of the previous month.
The monthly billing is extremely convenient for customers who make a lot of payments per month. Customers can enable this via their customer panel at https://www.versio.eu/customer. Click 'Your invoice' -> 'Monthly billing' on your left.