Meaning of TEMPLATE=aux in a qmake project file
Google returns "- Implemented "aux" template that allows making use of the INSTALLS variable without building anything. Needed for projects with QML entry point." From Digia's 4.8 release notes.
Also of interest: the patch which added this.
The TEMPLATE variable's value of aux is now documented at https://doc.qt.io/qt-5/qmake-variable-reference.html#template .
I have seen TEMPLATE = aux used in .pro files for Qt Installer Frameworks projects which create standalone executables which do not have to be run from Qt Creator.