What is a "Broker Broker's Trade" in the municipal bond market?

I was looking at the historical trades of a municipal bond using the Municipal Securities Rulemaking Board (MSRB) Electronic Municipal Market Access (EMMA) website:

Notice that the first row in the table has a special condition "B". Hovering the mouse pointer over the "B" reveals:

Broker’s Broker Trade: Inter-dealer trade by broker’s broker.

According to the help page:

Special Condition Indicator – shows whether any special conditions apply to the trade. Most trades do not have a special condition indicator. The categories of special conditions are as follows:

[…]

– (B) Broker’s Broker Trade – indicates an inter-dealer trade done by a broker’s broker.

Can someone explain what this means? What is an "inter-dealer trade done by a broker’s broker"? Why is there a "B" flag for the inter-dealer trade on the first row of the table, but not for the inter-dealer trade on the third row of the table?