Bound modifier

A bound modifier is a word, phrase or clause that cannot be moved or removed without changing what the sentence means, because it identifies which thing is being talked about rather than adding detail about it.

The drafts she had annotated went back in the folder. Lift the bold clause out and the sentence now claims that all the drafts went back, which is not what it said. Move it elsewhere and it means something else again, or nothing. The modifier is bound: it is welded to the noun it identifies.

The punctuation tells you which kind you are looking at. Bound modifiers take no commas; free modifiers take them. That is not a style preference — it is the reader's only signal as to whether the phrase is narrowing the field or decorating it. Compare the restrictive and non-restrictive versions under relative clause and the difference is a fact about how many drafts were in the folder.

Most single-word adjectives before a noun are bound too, and so is a restrictive appositive phrase: in my sister Ruth, the name identifies which sister and takes no commas.

The practical use of the term is in editorial notes. "This modifier is bound" means do not move this and do not comma it off — the sentence is relying on it to say which one.