Monolayer decorative film is generally made of a thin transparent plastic film (max about 150 microns) gravure-printed.
The transparent plastic is most of the time used as a protective top layer and can be made of different material : PS, PMMA, PVC …

Multilayer decorative film is generally thicker than monolayer decorative film (min about 200 microns)
Multilayer decorative film is a monolayer decorative film laminated with a substrate layer. The transparent plastic film(protective top layer) can be made of different materials : PS, PMMA, PVC …
The substrate layer can also be made of different materials : PVC, ABS, PS …