My question concerns only p 48: what does grammatical mean? Its use does not appear to match any of the definitions of 'grammar' below. I know of the distinctions between Descriptive, Prescriptive, and Teaching Grammar, but which do not appear relevant here.
Sorry for the surfeit of quotes. I thought to include more than less, but please tell me if I should remove any.
Source: An Introduction to Language (10 ed, 2014) by V Fromkin, R Rodman, N Hyams
[p 48:] The grammatical relation of a noun in a sentence is called the case of the noun. When case is marked by inflectional morphemes, the process is referred to as case morphology.
[p 558:] case A characteristic of nouns and pronouns, and in some languages articles and adjectives, determined by their function in the sentence, and generally indicated by the morphological form of the word [...]
[p 8:] [...] what does it mean to know a language? It means knowing the sounds and meanings of many, if not all, of the words of the language, and the rules for their combination—the grammar, which generates infinitely many possible sentences. We will have more to say about these rules of grammar in later chapters.
[p 25:] Speakers use a finite set of rules to produce and understand an infinite set of possible sentences. These rules are part of the grammar of a language, which develops when you acquire the language and includes the sound system (the phonology), the structure and properties of words (the morphology and lexicon), how words may be combined into phrases and sentences (the syntax), and the ways in which sounds and meanings are related (the semantics).
[p 566:] grammar = The mental representation of a speaker's linguistic competence; what a speaker knows about a language, including its phonology, morphology, syntax, semantics, and lexicon. A linguistic description of a speaker's mental grammar.