B15 commissions are now old, the new criteria are b30. B30 means beyond 30, cities other than Top 30.

For commission structure of mutual fund agents, AMFI has prescribed a commission structure, which is available to all registered commission agents. Under that structure, there has been a division among cities Tier I, Tier II, Tier III.
T30 (Top 30), includes all Tier I cities, and rest all the cities falls under B30 (beyond 30) both Tier II and Tier III cities. In these cities, mutual funds awareness and penetration is low, therefore to promote investment there, some additional incentive is given to agents in these cities. Hence, the need for separation arises.