What's the term for a comma that commences non-restrictive clauses?

You pine to use a comma, to clarify that John has just one ball. Is there a term for the mandatory comma that I embolded?

In each of these examples, the phrase set off by commas can
be eliminated without destroying the meaning of the sentence.
Remember, however, that there are some phrases that do not merely
describe a quality of a noun or verb but instead positively identify it.
An identifying phrase should not be set off by commas. To take an
earlier example, the following sentence includes a descriptive phrase
that can be eliminated without damaging the sense of the sentence
and therefore can be set aside by commas:

John threw the ball , [What’s this comma called?] which was coming apart at the seams.

If you wish to indicate that John picked one particular ball out of a
selection of many, you would delete the comma, writing:

John threw the ball which was coming apart at the seams.

Stacie Strong. BA English literature (UC Davis 1986), MPW (USC 1990), JD (Duke 1994), PhD Law (Cambridge 2002), DPhil (Oxford 2003). How to Write Law Essays & Exams 5 Ed 2018. 149.