How Copy/Paste works:

All of the methods use window properties to attach data, called a selection, to a source window. The program advertises the selection by using the X server. The window property used determines which selection type is set/advertised by the new selection.

When a paste is used in a target window, the target program requests the advertised selection data. This may access one of two buffers depending on which type of load/paste action is used. The user loads a cut buffer via Drag select or Ctrl-C/Ctrl-X, and pastes a cut buffer via middle mouse press or Ctrl-V.