Difference between mutual fund and brokerage

I have several mutual funds, most of which are defined as "Brokerage" type. I created  a new one defined as Trust (the fund is in my revocable trust) with "Single Mutual fund" enabled. When I sold some shares in the "Mutual fund" and downloaded the Sell transaction, it shows the dollar Amount, but the Balance is zero. When I sell shares in a "Brokerage", it shows the dollar amount and that amount gets added to the Balance. The Mutual fund transaction in real life transferred the amount into my Checking account. But when I try to edit the Sell txn to add Transfer info, it doesn't allow updating the Transfer field. When I try to add a Payment txn to the Mutual fund to show the transfer to the Checking account, it automagically saves that Payment as a Buy for $0.00, and gives me a popup (attached). I guess I can edit the Mutual fund account to change it to a Brokerage, but I'd like to understand what the Single Mutual Fund account attribute is and why it differs from a Brokerage type. Can't find any explanation in Help.