CA/B is neither a regulator nor, formally, a Standards Development Organisation although if you can squint it can look a little like either. CA/B produces the Baseline Requirements and also a set of EV Guidelines, for how a CA should behave.
The browser vendors (actually mostly OS vendors in their guise as browser vendors, Mozilla effectively stands in for all the Free Unix type operating systems) operate trust roots, and each of them decides their own criteria for things to appear in those roots.
BUT there was concern as the root store programmes began to tighten up their criteria that they might step on each others' toes, CA/B is a forum to share common principles. For example, rather than Mozilla tell CAs to stop issuing certificates that last more than 2 years plus a few days, and Google tell them not to issue ones that last more than 25 months, and then Microsoft say they want a rule forbidding new certificates that last more than 750 days, everybody got together at CA/B and passed a Baseline Requirements change setting the limit at 825 days from next year.
CA/B looks a lot like a cartel (all the companies in an industry meeting up to agree things) so it has a bunch of things it mustn't do to ensure it doesn't break anti-cartel legislation and get anybody sent to jail [The world's most famous cartel, OPEC, doesn't fear the law because OPEC is run by governments and governments have sovereign immunity]. For example it can't talk about prices, or future products.
Another result of not being a cartel is that CA/B doesn't bind either CAs or Browsers. The CAs can break CA/B rules without being kicked out of CA/B and the Browsers can and do require things beyond the Baseline Requirements, for example Microsoft requires every CA in its programme to revoke any certificate it tells them to, on demand.
Initially CA/B was all about smoke-filled rooms (maybe not literally) but Mozilla and Google have pressured them to gradually act more in the open, so you can actually follow along most of their decision making in real time via their mailing lists.