Dear Aspirant,
Compartment and Improvement both are different cases.

When a student fails in one subject then he or she will have to give the COMPARTMENT exam. In this exam student gets a chance to pass in that subject in which he or she got COMPARTMENT, 3 chances are given to the student for clearing COMPARTMENT one is in that year in which he gave boards in the month of July and second chance is given i think in Feb or March not sure and the other one is in the next year in the month of JULY if he or she doesn't pass in any of these three chances then he or she will have to repeat class 12th. If a student fails only in one subject then he or she will have to give COMPARTMENT EXAM but if he or she fails in more than 1 subject then it will consider as FAIL.

In the case of IMPROVEMENT student gets a chance to reappear for BOARDS in the next year as a private candidate but here is the conditions, For eligibility to give IMPROVEMENT student should pass in all subjects if he or she got COMPARTMENT in any of his or her subject then firstly he or she will have to clear the COMPARTMENT exam then he or she will be eligible for IMPROVEMENT.

Hope this information helps you,

Feel free to ask,

Thank you .