agulani

Think about it this way:
Principal trade: Broker has a bond in their inventory that they bought for $980. They mark it up to $990 and sell it to the client. Part of that $10 is a commission that generated soft dollars. It is harder to figure out the commission and soft dollars on the bond since it is an over the counter market. For a long time, brokers just completely ignored it, because it was not as easy to separate out.
Agency Trade: A stock is trading at $50. You buy at $50 and pay a $1 commission. The commission generates soft dollars. In an agency trade, it is very clear what part of the transaction is commission, and if you know what the soft dollar policy is, exactly what % goes to soft dollars.