Hi Nayana.

After filling the application form and registering you will get an email mentioning your candidate ID which you need to keep for future use.

After the successful completion of the Basic Registration process, the Code Generation will start. In this, candidates whose details submitted in Basic Registration is accepted will be given the facility of generating code for final registration. The facility of code generation was re-opened from February 23 till March 12 for the candidates who were not able to generate the code for some reason. Only those candidates who have generated the code for payment of fees and exam city choice will be eligible to proceed with the next step of AIIMS MBBS 2019 application form.

Hope this helps.

For more insight please check 

https://medicine.careers360.com/articles/how-to-fill-aiims-mbbs-application-form

Good luck!!