To my understanding, the Xfce Clipboard manager has the option to merge
the 'X' clipboard with the 'selection' clipboard.
When selecting text in X, the selected (or copied with ctrl-c) text is
immediately paste-able in an xterm using shift-insert.
The other way around: Selecting text in an xterm isn't paste-able in X,
unless explicitly selected from the 'Clipman History'. The 'Clipman
History' also shows 2 active clipboard items... (see attachment)