This one simple at first glance and will stay simple for objects that are supposed to look new and are made of plane, unaltered or painted metal. It is a greyscale map representing float values. White means the surface is metallic. Black means the surface is a non metal surface. Painted metal is also a non metal surface and should not contain any metallic values. With weathered and used objects , with some wear and tear, scratches and crevices will reveal some of the metal underneath, which would mean that the metallic map should be grey orr white, wherever some of that metal reveals. NOTE that when using the metallic workflow, as opposed to specualr/glossiness workflow, all non non metallic surfaces have a hard coded specular value of 0.04 in them, as this is appropriate for most non metallic surfaces in the real world. This makes them metallic workflow much simpler to stay in the bounds of creating physiically accurate results, but is also a bit limiting artistically, as it prohibits setting a surfaces specualr value directly. There are certain surfaces, especially cloithing like silk, where metallic values can be used to mimic the specualr effect. With silk, a value of about 0.5 metalness would be fine for example.