What are you talking about? X Windows has three¹: CLIPBOARD, PRIMARY, and SECONDARY. https://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM/icccm.html#pe

What are you talking about? X Windows has three¹: CLIPBOARD, PRIMARY, and SECONDARY. https://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM/icccm.html#peer_to_peer_communication_by_means_of_selections https://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM... Note the copyright dates on that document. ¹ Actually, there are as many as you care to implement. These three are simply conventional; applications are free to use other names to identify other communication channels.