Go to Accounting -> Go to Customer Contracts -> create or select any contract.
You can now manage the Period Performance of the contracts.
Use performance period on invoice is to turn off the function
The Start and End Current Period are calculated based on the fields Invoice Every/ Date Start
Invoice Every last day, the check means that the current period end always takes the last day of the month (e.g. checked start: 15.12.2022 end: 31.12.2022 / not checked start: 15.01.2022 end: 14.02.2022)
When the invoice is created, the period information is transferred to the invoice from this contract
The period duration can be used in the line items for the calculation
In the lower part of the window is a description if how it works