Hello Nadiya, Congrats on your eligibility. You can check out the following link for detailed info on AIIMS MBBS counselling: https://medicine.careers360.com/articles/aiims-mbbs-counselling

The mock round is just held to make candidates aquainted with the counselling and admission process and give them an idea on seat alottment based on choices made in the application. You can still participate in the 1st round directly.

Hello dear candidate. 

Participation in mock round isn't a compulsion. It just gives you a rough idea as to where you might stand under for the first round.

You can definitely take part in the first round of choice filling. 

I wish you luck for your future endeavors.