If you look up 'Best Execution Policy - CommSec' it will explain...

If you look up 'Best Execution Policy - CommSec' it will explain how to it all works. A useful excerpt from the document that addresses your curiosity might be as follows:

"For both retail and wholesale client orders, we will choose a venue or venues that satisfy our Best Execution Obligation. Execution venues that may be considered by CommSec when executing orders are ASX TradeMatch, ASX CentrePoint and Chi-X Australia. CommSec does not operate a crossing system."

Hope that helps somewhat SynQuasi, it is basically a legal requirement enforced under ASIC's Regulatory Guide 265.