Hii Debaraj!
Normally, in most of the schools across India, PCM or PCB is considered the main subjects, and one of the subjects in PCMB combination is optional or side subject. But, if you want to go in medical field in future and will opt in exams such as neet then, you ought to have PCB as your core or main subject. In case of IIT JEE you need to have PCM as your core combination because in the application forms you will have to enter the marks of the respective fields. So decide accordingly about your career. Hope you understand.
All the best!