ViewAR applications use the Right-handed Cartesian Coordinate System (see the graphic below).
Long story short: the Up-Axis is the Y-Axis!
The three axes defining the three-dimensional space of the Cartesian Coordinate System have two possible orientations. Therefore, there are two types of the system: Left-handed and Right-handed.
The ViewAR System uses the Right-handed Cartesian Coordinate System, meaning that the x axis goes right, the y axis goes up, and the z axis runs towards you. Keep it in mindand verify your settings before export.
If you are interested in learning more, this Wikipedia article should be a good starting point.