def,
In layman terms, Reg.T cash account part can be restated:
220.8 (1) Buy any security if
(i) you have the cash to buy it
OR
(ii) you don't have the cash, you can still buy it in "good faith"(with no cash) provided that you deposit the cash before you sell it.
Therefore, (ii) applies only when you buy securities without having the funds in a cash account.
IB does not allow you to buy securities without having the funds, hence (ii) does not affect IB and can not be any problem to IB.
If you have the cash, you can buy the security (from (i)).
Many are also confused by T+3, because it's frequently brought out of context as well as the buzzword "settled".
"Settle" means complete. To "settle" a transaction means to complete a transaction. (SEC,T+3:
http://www.sec.gov/answers/tplus3.htm)
SEC states:
"T+3 means that when you buy a security, your payment must be received by your brokerage firm no later than three business days after the trade is executed."
IB requires you to have the cash before you purchase a security, therefore when you buy a security, IB immediately receives the payment and the transaction is settled(=completed) instantly, in realtime. (You receive the money, they receive the cash)
When you sell a security from IB cash account, IB immediately deposits the proceeds* from the sale in the cash account. This completes (=settles) the transaction in realtime. And the cash is available for the next intraday purchase.
When you have the cash, regulation T allows you to purchase securities with it in a cash account.
In this way, you can continue your intraday trading in the cash account. (and complete hundreds of transactions per day if you wish). There's NO prohibition to do that either from Regulation T or the T+3 settlement cycle.
Fohat
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* or the (initial purchase amount- commissions to buy and sell) if IB doesn't receive immediately a cash payment for the sold security.