Hello aspirant,
I am here for helping you......
As you were asking about registration slip issued by jamia milia islamia for btech registration what basically it is ?.
So , it is basically a proof for you that you have registered in this college for that particular course , by default if college lost your name from their record then at least you can claim there and can go for the  further stage .
For knowing more about jamia milia islamia about it's rating , ranking , recommendation , placements ,etc you can go the link which I am providing you below:
https://engineering.careers360.com/articles/jamia-millia-islamia-jmi-btech-admissions .
I hole it will be beneficial for you.........
For more queries you can upvote my answer and can ask in the comment section.........
Best of luck.........