A phonological manifestation of a category value (for example, a word ending that marks "number" on a noun) is sometimes called an exponent.
Grammatical relations define relationships between words and phrases with certain parts of speech, depending on their position in the syntactic tree.
However, purely grammatical features do not always correspond simply or consistently to elements of meaning, and different authors may take significantly different approaches in their terminology and analysis.
Exponents of grammatical categories often appear in the same position or "slot" in the word (such as prefix, suffix or enclitic).
Categories can also pertain to sentence constituents that are larger than a single word (phrases, or sometimes clauses).
In traditional structural grammar, grammatical categories are semantic distinctions; this is reflected in a morphological or syntactic paradigm.
[1] For structuralists such as Roman Jakobson grammatical categories were lexemes that were based on binary oppositions of "a single feature of meaning that is equally present in all contexts of use".