Being new to MD, I have just discovered the issue with sweep accounts. This is pretty much a game stopper for anyone with large brokerage firm accounts, or with mutual funds that are held in accounts separate from their Brokerage Account -- usually an IRA. HERE IS WHAT I KNOW ABOUT SWEEP ACCOUNTS:
1. Most major brokerages use either a bank account, or a money market account to keep track of cash not in use elsewhere in a portfolio. They "sweep" any cash balance by transfer into this account, and draw from it when a asset (Stock, Bond, Certificate, etc.) is purchased. This account has a different account number than any Brokerage Account, or in some cases such as T Rowe Price, any Mutual Fund.
2. In the securities account (Brokerage or Mutual Fund), the cash is received, and the purchase made, which results in a zero cash balance in the particular securities account.
3. The "sweep" account may not have a cash balance either (but may), as any incoming cash is used to buy some Money Market instrument such as Treasuries. For those that do keep a cash balance, they are always interest bearing (tho not much).
4. A few companies (T Rowe Price being one) use the brokerage sweep account for cash related to Mutual Fund Accounts that are NOT a part of the brokerage account. If you buy, sell, or transfer within the Mutual Funds, or between the Brokerage Account and the Mutual Funds, the cash part of the transaction passes through the sweep account with any difference winding up there.
5. On some rare occasions, usually when a customer has brokerage or mutual fund accounts with more than one company, the sweep account may be held in an independent bank account. This NEVER happens in an IRA due to Government rules.
6. In almost all cases above, deposits received and distributions made to other locations ALWAYS come into, or out of, the sweep account.
7. In a few rare cases, distributions may be made directly from a Mutual Fund Account, this is rare, and only a few companies do it.

From a design perspective, this can all be done with linked accounts, but special rules must be implemented that, in effect, treat the cash balance part of the investment account differently for linked accounts. It would also be useful to have the ability to group multiple accounts, with different account numbers under one online access user account (T Rowe Price, USAA, and several others) for investment reporting purposes.