In the inspector, you have a component called "Part" 

This component references all the original information regarding the occurrence imported with some geometrical information. In Pixyz Review, you can disregard all these information except the Patches section.

Patches on occurrence

As you might see, occurrence have sometime "No Material" as a property but in the viewer, we can still see a specific materials. 

This situation happens when the "Part" (the origin of the occurrence) has already materials applied on it. Patches refer to a group of polygons that constitute specific entities of the part. We could called them "poly-group".

For instance, on a 3D laptop model, it may occurs that one occurrence will define the full laptop, but that in this occurrence, the screen would be 1 patch, the keyboard another one and the structure another as well). It'd allow to apply different materials to one occurrence which wouldn't be possible with the "Material" property of the whole occurrence.