Hi Rupam,
As per PSCD functionality, if you do not have contract object in your master data set up, the contract account category becomes the account determination ID.
So, for any financial posting, where you want to post at CA level, system retrieves the CAC and uses that as Account determination ID before hitting the posting specific posting areas.
It all happens in FQEVENT 2100. You can put breakpoint in this event while document posting and see the account determination characteristic.
Hope it helps.
Thanks
Bodhisattwa