As part of resolving bugs where there's complex systems at play or the cause is not immediately obvious, a developer may ask you to send you an RPT file.
Click in the address bar
Paste %LocalAppData%\Arma 3 and press your Enter key
The address bar should now show an Arma 3 folder underneath AppData in your user account
RPT Files are named by the time arma started, send the file for the session where the bug occured