Hello aspirant
Answering your question-
15% quota is the All India Quota Seats which means 15% Seats of All 460 Medical Colleges in India(Including Private, Government)will be filled via Centralised All India Counselling for the 15% Seats which means an Institution whether Government or Private will have to reserve 15 % of its seats which will be filled via All India NEET Counselling.

And the 85% quota is the seat reserved for the Students in their state.

Suppose, a government medical college named 'X medical college' has 100 seats for UG.

So out of this 100 seats, 15% i.e. 15 seats will be filled through all India quota. That means 15 seats can be occupied by students from any state all over India except a few states where this rule is not applicable.

And remaining 85% i.e. 85 seats will be filled only by the students from that particular state in which 'X medical college' is situated.

And if you consider all the medical colleges of India, then 15% of all seats is filled by any student who qualified NEET and 85% seats is filled by students from that state and have a domicile for that state..
Hope this helps you.