TL;DR
- The exporter will create a .glb model of geometry visible in your active 3D view.
- The origin of the exported .glb can be specified by including a AR_QR_Code.rfa in your view. Download AR_QR_Code.rfa
Setting the GLB Origin
You can define a custom origin by including the Revit family called "AR_QR_Code.rfa" in your 3D view. The exporter looks for one of these families in your view and will export the geometry such that this becomes the 0,0,0 of your GLB model.
Visual Style 
- Realistic or Textures: Exports material textures from the Appearance tab*
- Consistent Colors or Shaded: Applies the color from the material's graphics tab
- Hidden Line or Wireframe: Exports all geometry as white
*Not all appearance schemas are supported. If a material is not exporting correctly to your .glb create a new material and use an image that is located in your Custom Textures folder.
Texture File Locations
The exporter automatically searches for texture files in C:\Program Files\Common Files\Autodesk Shared\Materials\Textures\
The Custom Textures button will add and open a subfolder to the above location. Save custom textures here and the exporter will be able to find them.
Don't host textures on networked drives or file storage services like Dropbox.