Thank god for the manual!
Is on mobile device
Tries to detect whether the current device viewing the project is a mobile (e.g. phone or tablet). It is difficult to accurately detect this so detection may not be perfect: there may be some mobile systems for which Is on mobile is not true, and (although less likely) desktop systems for which Is on mobile returns true. The condition always returns true when packaged via the PhoneGap/Intel XDK/CocoonJS containers, and always returns true for iOS, Android, Blackberry 10, Windows Phone and Tizen based devices. Beyond that Construct 2 will test a few other common mobile manufacturers. Otherwise, it will return true if the current system is not a desktop system, since there are relatively few desktop OSs; the desktop systems it checks for are Windows, Linux, OS 9, OS X, Solaris and BSD.