The actual Reg T rules for cash accounts are relatively subtle. They prohibit buying, selling, and then buying again using the same funds before the original purchase has settled. The sale does not have to settle before the funds can be used. You can buy and sell on the same day, then use those funds three days later. You can buy, sell three days later, and then buy again immediately, even though the sale has not cleared. The only requirement is that you do not make more than one purchase with the same funds before the first purchase has settled. If you do, it is called a free riding violation and results in a 60 day restriction on your account.
Various brokers implement this rule in different ways. Many brokers, including Interactive Brokers and BrownCo, incorrectly require all transactions to settle in cash accounts, not just purchases. Some brokers have been caught allowing their customers to violate Reg T entirely. A few brokers, including Fidelity and FOLIOfn, correctly implement the Reg T requirements without enforcing additional and unnecessary house requirements on cash settlement.
Martin