A category, in the sense relevant here is a class or division in general scheme of classification.
In modern work the work ”„feature”¦ is used for a set of mutually exclusive properties of words or phrases. A feature NUMBER might be postulated with singular, dual, and plural as its possible ”„values”¦. A feature paired with a value can be called a ”„feature specification”¦. (Intuitively, NUMBER is like a question to which ”„singular”¦, ”„dual”¦, and ”„plural”¦ are the different possible answers, and a feature specification is like a question paired with an answer.) The notation F:v will be used in this article to indicate that the feature F has the value v. Thus the feature specification NUMBER:plural would be a grammatical device to indicate that the feature NUMBER has plural as its value, hence that the expression having that feature specification as part of its grammatical representation is a plural noun rather that a singular or dual one. In phonology a different notation for feature specifications is familiar; ”„[+VOICE]”¦ would be used where the notation in this article uses ”„VOICE:+”¦ to indicate that VOICE has the positive value ”„+”¦.