Suppose a student is from state A. Does he/she can apply for other state quota in NEET UG counselling?

Depends on whether it is For Government Medical Colleges or Private Medical Colleges. 15% of all the seats are filled through All India Counselling and so you can participate in All India Counselling of GMC seats of other States. But for seat in Private Medical Colleges, the process is different. Private Medical Colleges do not participate in All India Counselling and the seats are filled through State Counselling alone. And for appearing for State Counselling you need to hold the domicile of that particular State. Or else the only other option is through Management Quota. Under All India Quota, All the 15% of All India Quota Seats except for Jammu and Kashmir, 100% Seats at Deemed and Central Universities, IP Quota seats in ESIC Colleges, Seats at AFMC, AIIMS and JIPMER are filled by DGHS. The 85% State Quota and Private Colleges are filled by the respective State Authorities. And there are 3 Categories in Private Medical Colleges. Category A is the Government Quota Seats. And only candidates of the home State are eligible to apply for these seats. B Category seats are also called Management Quota Seats and they are filled through State Counselling and the remaining seats are filled through direct Admissions by contacting the College directly provided the Candidate has registered for the respective State Counselling and has not been able to secure a seat in the Counselling. this is where you can apply for. Here the fees is usually nothing less than 40 lakhs. The last Category is NRI seats Category and you cannot apply for that.