This section concerns X-Window users who are used to cutting and pasting with the middle mouse button. In the GPS text editor, as in many recent X applications, the GPS clipboard is set by explicit cut/copy/paste actions, either through menu items or keyboard shortcuts, and the primary clipboard (i.e. the “middle button” clipboard) is set by the current selection.
Therefore, copy/paste between GPS and other X applications using the primary clipboard will still work, provided that there is some text currently selected. The GPS clipboard, when set, will override the primary clipboard.
See http://standards.freedesktop.org/clipboards-spec/clipboards-latest.txt for more information.