You can still sign on JPSDS after completion of NS (NSF). After
completion, you will go for your studies in Poly and after that,
you will be a Regular in SAF. But you can't leave anytime. This is
because you have a bond with SAF. If you want to leave anytime, you
have to pay the liquidated damages to SAF before you are able to
leave.
If you are discharged due to poor performance, you still have to
pay. If you are discharged due to service injury, etc, then u will
leave without paying. Each case is case-by-case basis.