It's how T2 settlement works. If you trade the same amount or more than the previous day your settlement date will be pushed out because commsec allow you to buy against unsettled trade credits from a previous sell on the same day.
Commsec also allow you to trade up 7.5k on specs without actually having the money in your CDIA account. Up to 25k on blue chips but you have to keep a good credit rating with them ie. Don't default in settlements as it will affect your rating.
I believe commsec allow you to trade up to 50 percent of the value of your current positions in your commsec account over and above the 7.5k initial threshold
Please DYOR on this though
Not many brokers allow this. Hence why I believe they are quite popular amongst day traders...
Ps be careful with these features. It's a very easy way to do your cash. Especially if you get stuck in a trading halt and you cant cover your trade. Ie don't trade what you cant afford to settle.
