Consider the Latin phrases in (38):

In each of these phrases the adjective and noun agree in number, gender, and case. As you can see, the adjective ‘good’ can occur in any of the three genders, depending on the context in which it occurs. The nouns ‘boy’, ‘girl’, and ‘gift’ are always, however, respectively masculine, feminine, and neuter nouns. In other words, the inflectional category of gender is inherent in nouns, whereas it is contextual in adjectives.

Put more generally, contextual inflection is inflection that is determined by the syntactic construction in which a word finds itself, whereas inherent inflection is inflection that does not depend on the syntactic context in which a word finds itself. Number is inherent in nouns and pronouns, as is gender. But the case of a noun always depends on its syntactic context. On the other hand, tense and aspect are inherent in verbs, but person and number depend on the nouns or pronouns with which a verb occurs in a sentence. So number can be contextual for one category (verbs) but inherent for another (nouns).