Hello candidate,

Seat allotment letter is issued to the candidate by Joint Seat Allocation Authority (JOSAA) when the seat is alloted to him/her in the month of June or July. It is the process after declaration of result of different rounds of counselling. Allotment letter is only issued when you are eligible for the seat. You have to download it from the official website of JOSAA.

Hope this answer cleared your doubt.

Good Luck!!