After a Paste, use the Paste Control popup menu to control how the image currently being pasted is transferred to the destination image. Except for Blend and Transparent, the Paste Control transfer modes are the same as those listed in the description of Process>Image Calculator.
The Blend mode is the same as the Image Calculator's Average mode.
In Transparent white mode, the white pixels are transparent while all other pixels are copied unchanged.
In Transparent-zero mode, the zero pixels are transparent while all other pixels are copied unchanged.
Transparent and Transparent-zero modes only work with 8-bit and RGB images.