Surface Materials in 3DCoat are the equivalent of polygonal material assignments from other programs, such as Modo or Maya. If upon import 3DCoat detects any of these Surface Materials, they will be respected and kept, and can also be used to perform various functions in the Paint Room.
Surface Materials are also created in the Retopo Room, when Baking a Retopo Object to the Paint Room, The Retopo Objects name and polygons in the Retopo Object are assigned as a Surface Material, in the Paint Object. This can only be performed on Retopo Objects, however. Also note that this functionality is rudimentary, so if more advanced Surface Material assignments are needed, it’s best to export and perform the assignments to the mesh to another application, such as Modo, and then import back into the Paint Room for painting. Typically though, most Objects that are exported from 3DCoat tend to be game assets and having few Surface Materials is often preferred over too many.
Or if you wish to stay in 3DCoat, a more advanced technique is to retopologize the object with multiple Retopo Objects as you wish Surface Materials, with polygons for those Surface Materials, and having the bordering edges of these groups where they connect to one another to have overlapping vertices. By doing this, you can then Bake to the Paint Room, paint as normal, then export the Objects and textures to another application, and perform a weld vertices operation to connect the mesh. This is of course, if you wish all of the objects to be connect as one.