Can be both yes and no... 
The medical checkup in CMPB would determine your PES status... and
this would subsequently decide your enlistment, posting, vocation
etc... If you have gotten PES A, B, C or E, it's confirmed that
you'll have to serve NS... unless you have been graded PES F
(Exempted From NS)...
Note: Enlistment would usually be around 6 to 9 months after your
medical checkup...