Yes - it's your back up for whatever reason you may not be able to do medicine e.g. Your grades or something. Or if you don't want to do medicine anymore.
Universities won't discriminate you for using a medicine ps.
Yes you can apply by 15 Jan after your medical applications.
I would recommend you apply to QUML for biomed as if you are one of the top 9 students, you're guaranteed a place in medicine. This is very useful if you don't get the right A-level grades and have to go graduated route.