@aurimasv: MSC fields are a sort of keywords, yes. See
    http://www.ams.org/mathscinet/help/field_help.html#msc
There is one twist tough: every record in the MathSciNet database (Zentralblatt too, maybe others) is assigned a primary MSC code, and possibly one or more secondary MSC codes.

I'd find useful if they were imported as Zotero tags (easier to find items), but that way the information of which MSC code is the primary one and which ones are secondary is lost.