Halliday & Matthiessen (1999: 26-7):
The understanding of covert categories in the grammar is due to Whorf (1956: 88 ff), who made the distinction between overt categories or phenotypes and covert categories or cryptotypes; he is worth quoting at some length:
An overt category is a category having a formal mark which is present (with only infrequent exceptions) in every sentence containing a member of the category. The mark need not be part of the same word to which the category may be said to be attached in a paradigmatic sense; i.e. it need not be a suffix, prefix, vowel change, or other 'inflection', but may be a detached word or a certain patterning of the whole sentence. ...
A covert category is marked, whether morphemically or by sentence pattern, only in certain types of sentence and not in every sentence in which a word or element belonging to the category occurs. The class membership of the word is not apparent until there is a question of using it or referring to it in one of these special types of sentence, and then we find that this word belongs to a class requiring some sort of distinctive treatment, which may even be the negative treatment of excluding that type of sentence. This distinctive treatment we may call the reactance of the category. ...
A covert category may also be termed a cryptotype, a name which calls attention to the rather hidden, cryptic nature of such word-groups, especially when they are not strongly contrasted in idea, nor marked by frequently occurring reactances such as pronouns. They easily escape notice and may be hard to define, and yet may have profound influence on linguistic behaviour. ... Names of countries and cities in English form a cryptotype with the reactance that they are not referred to by personal pronouns as objects of the prepositions 'in, at, to, from'. We can say 'I live in Boston' but not 'That's Boston — I live in it'.