The grammatical categories which are realised by the described types of forms organised in functional paradigmatic oppositions, can either be innate for a given class of words, or only be expressed on the surface of it, serving as a sign of correlation with some other class.
For instance, the category of number is organically connected with the functional nature of the noun; it directly exposes the number of the referent substance, e.g. one ship — several ships. The category of number in the verb, however, by no means gives a natural meaningful characteristic to the denoted process: the process is devoid of numerical features such as are expressed by the grammatical number. Indeed, what is rendered by the verbal number is not a quantitative characterisation of the process, but a numerical featuring of the subject-referent. Cf.:
The girl is smiling. — The girls are smiling. The ship is in the harbour. — The ships are in the harbour.
Thus, from the point of view of referent relation, grammatical categories should be divided into "immanent" categories, i.e. categories innate for a given lexemic class, and "reflective" categories, i.e. categories of a secondary, derivative semantic value. Categorial forms based on subordinative grammatical agreement (such as the verbal person, the verbal number) are reflective, while categorial forms stipulating grammatical agreement in lexemes of a contiguous word-class (such as the substantive-pronominal person, the substantive number) are immanent. Immanent are also such categories and their forms as are closed within a word-class, i.e. do not transgress its borders; to these belong the tense of the verb, the comparison of the adjective and adverb, etc.
Another essential division of grammatical categories is based on the changeability factor of the exposed feature. Namely, the feature of the referent expressed by the category can be either constant (unchangeable, "derivational"), or variable