Hi

surface film refers to prepreg that is generally only used for the first layer.  It is in general more resin rich and has a lot of resin on one side of the prepreg.  This done because because you need more resin than normal to get a perfect class A surface finish.  If you were to use a regular prepreg that is not intented to be used as a surface layer then you morethan likely will have a pin hole resin stared surface finish.  I hope that answers your questions.   

Cheers :-)