According to Fillmore, each verb selects a certain number of deep cases which form its case frame. Thus, a case frame describes important aspects of semantic valency, of verbs, adjectives and nouns.
Case frames are subject to certain constraints, such as that a deep case can occur only once per sentence.
Some of the cases are obligatory and others are optional. Obligatory cases may not be deleted, at the risk of producing ungrammatical sentences.

A fundamental hypothesis of case grammar is that grammatical functions, such as subject or object, are determined by the deep, semantic valence of the verb, which finds its syntactic correlate in such grammatical categories as Subject and Object, and in grammatical cases such as Nominative, Accusative, etc. 

Case grammar is an attempt to establish a semantic grammar. (Most grammars by linguists take syntax as the starting-point).