hi,

the compulsory subjects means the course or subjects you have to take mandatory and their is no choice or option to not selection of the subjects. however elective subjects are those which is given option to students to choose the stream or course or any particular subject as per their choice.

for example: for science stream majority of school has rules for 10+2 course students that, physics , chemistry and mathematics are mandatory.,students has option to select for fourth subjects from biology/ biotechnology/ computer science / statistics etc.

hope you may understand.