If by tiers you mean 4-, 3-, and 2-year scholarships, the answer is yes. My son received a 4-year Marine option scholarship this year but I don't recall it being based on what he plans to study in college.
Marine options scholarships are very competitive. I've heard anecdotally that for every six Navy option awards, there is one Marine option awarded. In our multi-state geographical area, somewhere around 50 applicants were qualified academically and athletically, and 18 received scholarships.
NROTC scholarships only cover tuition, uniforms, books, and mandatory fees. Room and board are not covered. It is up to the individual school to determine their room/board policy. My son's school provides a full room/board scholarship to ROTC scholars. Some schools don't cover anything.
Feel free to PM me if you have any more questions.