The MSRB writes conduct and disclosure rules for dealers and advisers in the municipal securities market, the bonds issued by states, cities, school districts, and public authorities. It does not examine firms itself: enforcement is done by finra, the sec, and bank regulators.
Its most useful product for an ordinary investor is EMMA, the Electronic Municipal Market Access system, where official statements, continuing disclosures, and actual trade prices for individual municipal bonds are published free. Because the muni market is quote driven and thinly traded, seeing real prints matters more than in equities.
MSRB rules also cover fair pricing and the markup disclosure that must appear on many retail muni confirmations, which made a previously invisible cost visible.
Related: finra, sec, self-regulatory-organization