Field help (DATENDPER) @ L.V. Expertise X3

It is the period end date.
This date must be higher than the period start date and less or equal to the fiscal year end date. The last period end date necessarily must correspond to the fiscal year end date.

The modification of a period end date is authorized only for periods with the status: Not open. It is used to modify the following period start date which automatically takes the value of the previous period end date + 1 day.
The closure of a period by right-clicking Close period, enables the user to split again the period Not open.
When the Fixed assets setup ACCPERCTL - Accounting period control is set to Yes, it is not authorized to modify the period breakdown directly ; this modification has to be carried out via the Context synchronization processing after having changed the breakdown of the periods in the Financials module.