Copolymer purity, what does AN / MA / MS mean?
I contacted GoodFellow directly and they gave me the following answer:

93.9% AN: 93.9% AcryloNitrile
5.8% MA: 5.8% Methyl Acrylate
0.3% MS: 0.3% Methyl Sulfonate

I guess different manufacturers may name their products as they see fit. In particular, for polymers that can share the same abbreviations, only the context and a good background knowledge of their synthesis pathway may allow one to decide...
There are, however, IUPAC guidelines for polymer abbreviations available here[1]. In addition several other websites offer their own abbreviation list e.g. here and there.

[1]. J. He et al., Abbreviations of polymer names and guidelines for abbreviating polymer names (IUPAC Recommendations 2014), 2014. DOI: 10.1515/pac-2012-1203