Paste Mix command pastes the content of the clipboard with the audio stream that we apply the action. Mixing is an addition of source and target samples of the two streams. 

In picture above you can see how we have paste mixed a part of the first audio stream to the second. 

Regarding sampling rate the source and target audio streams have to have the same sampling rate. If this is not the case the source audio stream sampling rate will be adjusted to the target sampling rate causing an undesirable effect. 

If the source audio stream is mono and the target is stereo the mixing will take place in both channels of the target audio stream. If the source is stereo and the target is mono the source audio steam paste mixed will be converted to mono. If both source and target with the same number of channels the mixing will be done from channel to channel 

The same result could be achieved with drag and drop operations. Right mouse clicking on the selected area, the mouse pointer changes to “drop pointer”. Once you drag it to the point of the target audio stream the mouse pointer changes to “Paste Mix” style showing that you can do the action. 

Releasing the mouse at that point you will have both streams mixed.