When exporting to the Oma riista service, you can choose either “Automatic removal of fragmented areas” or “Cut according to area marker” (this is selected in the “Oma riista export” view).
If you choose the latter, you can do this by opening the “Map markers” tab in the Reviiri service and creating a new “Area marker” there (open the Map markers tab -> select “Area marker” as the marker type -> click “+ New map marker” and create an area marker for the area to be cut.
After that, open the “Summaries” tab again, click the “Oma riista” button, and select “Cut according to area marker”, then choose the area marker you saved from the dropdown menu, which will be used for cutting the area.