Dear,
On the basis of possession letter issued by DDA office after depositing of demand money as well as required documents, you would approach concern JE of your area. The JE deployed on site would issue a physical possession letter. This possession letter will have to be submitted to DDA office for execution of Conveyance Deed.
The allotment letter would remain same however, till you get your flat freehold through conveyance Deed, allotment letter is irrelevant for future requirements, neither you can sale your flat nor you can apply for voter ID, Adhar etc.
Dilip Mukerjee