Personally, as a CommSec user, I find the offset / contra...

Personally, as a CommSec user, I find the offset / contra feature incredibly confusing (note: I come from the US, where BTW, there's only one Acct, not two, no separate Bank Acct for Cash). It makes it very hard to figure out exactly how much money you have to spend, you have keep a rolling computation of unsettled trades, and their sequence (with different sequences for BUY and Sell, as @roaminoz has noted). And then add linked Cash Balance.
Sometimes, I've called the general help line which is rarely helpful, (but I now have an excellent Senior contact there) and asked "How much do I actually have to Spend"...it takes them 5-10 minutes. On top of which I was given inaccurate information, several times in one week, on diverse subjects, including this one, as to how, exactly to calculate it.  I escalated it to my contact, and was told (by that same, very good guy), "don't call the General Line any more, just come straight to me".
I thought he was slapping me on the hand like a naughty child, not to hassle the 13 15 19 folks any more, and asked why. He was surprisingly blunt: "Because they've given you too much wrong information on too many matters in a week".
BTW here is his summary, sent to me  (I have excluded the Section on "Trading Limits"). I guess it's useful feature if you can understand it (contra).
CONTRA

Default Contra – Currently if you make a BUY order first and then SELL within T+1, the orders will offset. If you make a SELL order first and then BUY within T+2, the orders will offset

Day by Day Contra – All trades made within the same Business day will offset with each other and settle on T+3 with the net figure coming from/going to your bank account regardless of what happens on the days afterwards

Line by Line Contra – No offset or netting off, every BUY & SELL will be debited/credited separately from your bank account, so if you make 5 trades today (Friday) there will be 5 separate Direct Debits/Credits on Wednesday (T+3)

All clear now?