Paste Mix
This function takes the audio you have previous copied or cut and mixes it with the current selected region.
After you have cut or copied the audio you want to mix select the new region and used Edit -> Paste Mix from the menu (or use Ctrl+Alt+V). You can specify the volume of the mix (that is the volume of the clipboard audio).
Almost always the length of the selected regions will be different. If the mix selection (in the clipboard) is shorter than the current selection then the mixed audio will just end when it ends. If the mix clipboard is longer than the current selection then one of the follow can be selected. "Increase selection" means that the mix just continues past the end until the mix file ends. "Insert Silence" means at the end of the current selection, the mix will be inserted until it ends. The fade out mix means that as the end of the current selection is approach the mix fades out - this is useful for background music beds and SFX.
A typical example of Paste Mix is where you want to mix a music bed behind a voice over. To do this copy the music track from one file, open the voice file select the voice (say with Ctrl+A) then select Edit -> Paste Mix and fade out mix.
If the file you are pasting into is stereo you can select the stereo position of the file to be pasted. For example if you are paste-mixing a sound effect you can place that sound towards the left by sliding the pan fader left.
Audio can also be pasted from the System Clipboard (Ctrl+Alt+Shift+V). The System Clipboard can be used to copy and paste audio to and from other applications.