F13 is the PrintScreen (PrtScn) button. I think the original poster is cryptically pointing out a hotkey combination that prints the active window into your clipboard (instead of the entire screen). However, this can be done using the simpler Alt-PrtScn. To review:

* PrtScn: prints the whole screen into your clipboard

* Alt-PrtScn: prints the active window into your clipboard

* Ctrl-Alt-PrtScn: same as Alt-PrtScn