Check this box if the bolt load is transferred to the tubesheet, which is extended as the flange.
If the tubesheet is gasketed with both the shell and channel flanges, then tubesheet can still be extended but the bolt load is not transferred to the tubesheet extension. In that case, you can uncheck this box. But, carefully consider all the possible cases such the hydrotest.
If this box is unchecked then the required thickness of the tubesheet extension is not computed.