In this report you can filter on a specific date, showing you all open invoices on this date. 
This search is based on the field ‘Issue date’ on the invoice. For example:
You can also filter on period, showing you all the open invoices for this month. For example:
Please note: documents can have an issue date in for example 2021 but be posted in for example period 202012. If we have an invoice with issue date 2021.01.05 and period 202012, this invoice will be included in the report when filtering REP/101100;202012 but not when filtering REP/101100;2020.12.31 since the issue date on the invoice is 2021.01.05.
Please also note that you can filter on REP/101100;YYYY14 to include open invoice posted during the whole year, including period 13.

Similar report for open accounts receivable postings is REP/101098.