A primary bank account is basically the main bank account that is linked to your Kotak Securities trading account. When you submit the details of this bank account in order to open your demat and trading account, it is treated and linked as a primary bank account. 

It is used for both depositing and withdrawing funds to and from your trading account.

On the other hand, a secondary bank account is an extra bank account that can be added to your trading account. It can be used to only deposit funds in your trading account. Withdrawal to this account is not permissible. 

Please remember that you can add multiple bank accounts in your trading account, but only one will be considered to be as the primary bank account.