It would be useful to have an oportunity to use selected displays instead of all or one. Like FreeRDP has a /monitors:<0,1,2...> option.
This appears to be undocumented functionality in the Microsoft RDP client, demonstrated by Lu on https://www.youtube.com/watch?v=mVxzSY3qj_Y and written up in https://www.hanselman.com/blog/HowToRemoteDesktopFullscreenRDPWithJustSOMEOfYourMultipleMonitors.aspx
It would be useful if Royal TSX implements this. In the meantime I will try editing the connection document.
Unfortunately I haven't been able to get the hack to work with even the Microsoft RDP client for Mac, may be Windows only and am still tinkering. However a workable alternative for me on the LAN has been to enable full multi monitor support on the client and then just hide the unused spaces though this approach is obviously not as efficient.