The profit / loss can be allocated to various accounts here. For the date, December 31st is suggested. This way you'll have two closing balance sheets available in the reports:

Closing balance I (before profit allocation)
Closing balance II (after profit allocation)

The date can be changed, e.g. to January 1st of the next year. Please note that you will then not have a Closing balance II (because it will be the same as Closing balance I).

The fields Date, Description, Account and Currency are pre-filled. You only have to enter the accounts and amounts (Add / Edit).

At the bottom you see the profit / loss, how much has already been allocated and the difference (still to be allocated). Don't save until the difference is 0.00.

Upon saving, it will create a collective entry in the Journal. If you edit the book entry later in the Journal, it will show the collective entry dialog instead of this one.