A grammar type is a whole approach to describing or teaching a language, not one rule or mistake. A reference grammar describing major constructions counts, and so does a pedagogical grammar sequencing topics for learners. A rule like use fewer with count nouns does not.
Grammar types at a glance
Reference grammar describes a language's major constructions fully. Clue: it works like a lookup description. Example: A reference grammar of English might have a chapter on relative clauses covering every common pattern.
Pedagogical grammar teaches learners. Clue: it sequences topics as a course would. Example: it introduces greetings and everyday question forms before rare constructions. A real example: Emily Hawkins published a pedagogical grammar of Hawaiian in 1982.
Prescriptive grammar tells speakers what they should say. Clue: it judges forms as correct or incorrect. Example: advice to say It is I instead of It's me.
Descriptive grammar records how speakers actually talk, without judging. Clue: it reports usage rather than correcting it. Example: noting that many speakers say less books, and describing when they do.
Theoretical grammar builds or tests a general model of how language works. Clue: the focus is the theory, with one language as evidence. Example: a framework that analyzes English questions to explain universal properties of questions.
When the test needs refinement
Some books blend the two styles. The tendencies, not hard rules, are that reference grammars lean toward universal structural categories, while pedagogical grammars lean toward usefulness and ease of learning. The overlap is a matter of degree, not an absolute division: a teaching grammar may include reference chapters, and a reference grammar may add practice material. What still separates the kinds is purpose. If the book mainly answers the question "how does this language work in full?" it behaves like a reference grammar; if it mainly shapes the learner's path through the material, it behaves like a pedagogical one, even when individual chapters blur the line.
The diagnostic question: is the organizing principle the learner's path or the language's full structure? If the learner's path drives the book, classify it as pedagogical even when some chapters look descriptive in style.