A tagset is a list of part-of-speech tags (POS tags for short), i.e. labels used to indicate the part of speech and sometimes also other grammatical categories (case, tense etc.) of each token in a text corpus.
MULTEXT-East Morphosyntactic Serbian Specification is available in Serbian corpora. The MULTEXT-East resources are a multilingual dataset for language engineering research and development. See more at http://anthology.aclweb.org/W/W03/W03-2904.pdf