MSRB Rule G-17 (the obligation of an MSRB member to "deal fairly with all persons" and not to engage in any "unfair practice") is the MSRB's catch-all as to behavior that the regulators do not like, but as to which there is no specific prohibition. FINRA Rule 2010 (a member firm shall observe "high standards of commercial honor and just and equitable principles of trade") services the same purpose as to corporate transactions. Because the Rule is so open-ended, violations of the Rule likewise can relate to a very broad range of topics from underwriting conduct to interactions with retail customers.
It is sometimes the case that a particular type of violation of Rule G-17 will be viewed as sufficiently significant or common such that the MSRB still then adopts a more specific rule. For example, MSRB Rule G-47 ("Time of Trade Disclosure") is the direct descendant of various MSRB interpretations of Rule G-17.
See also the page on Muni Time of Trade Disclosures.