That is the defining, or restrictive pronoun, which the nondefining, or nonrestrictive.
The lawn mower that is broken is in the garage. (Tells which one.)
The lawn mower, which is broken, is in the garage. (Adds a fact about the only mower in question.)
Strunk and White, 3rd Edn, p59.
Some grammarians and authors don't make this distinction, but complete clarity is more important in mathematical writing than in other forms of writing.
For example, what does the following sentence mean?
Cayley’s proof was first for scrolls which are cones on a curve of degree n that have only cusps and double points (Historia Math., 33 (2006), p318).That Cayley's proof was for all scrolls or only for certain scrolls? In fact, the second. This would have been clear if the author had written:
Cayley’s proof was first for scrolls that are cones on a curve of degree n that have only cusps and double points.