When the clipboard contains raw image data or single layers, it's the same as the normal "Paste" (and "Paste In Place" respectively). These actions are useful if you want to copy a bunch of layers and paste them "merged" into a single layers (since now the copy-paste of multiple layers will create multiple layers). It is somehow similar to the "Copy Visible" action except that it works on selected layers only and work at paste time, making the action more versatile.
This directory contains the PDB-wrappers for various internal Gimp functions. THESE FILES ARE AUTOGENERATED AND CHANGES HERE ARE USELESS! If you need to change the code please have a look in ../../pdb/groups.