When you create and save a project defined report, the saved reports will also show in different room windows in dRofus so they can be used directly for e.g. an open room data sheet. If you open a RDS for a room and press "Print" you can see that the saved reports are added to the list;
Figure: Saved report in "Reports and exports" available directly in room RDS window 
Where the saved reports are shown depends on what type of report is being saved. E.g. all Room Data Sheet reports will show in the room RDS. All Template Room Data Sheet reports will show in the template RDS. Saved equipment reports will show in Equipment in room etc.