Difference between return from interrupt(RTI) and return from subroutine(RTS)

Usually return from interrupt restores the flags so that the interrupted code can continue to execute properly. Return from subroutine does not need to do that instruction is used intentionally in that flow of code and known that the flags are or are not destroyed depending on the architecture. In architectures that use a stack for the return address it is very apparent. A return from interrupt will pop the flags then the return address where a return from subroutine will pop only the return address.

When a hardware interrupt occurs on an x86 the flags and return code segment+offset are pushed onto the stack. Then interrupts are disabled. This is to set the stage for the interrupt routine to service the interrupt: switch stacks or whatever it wants to do before either re-enabling interrupts and processing some more before/or returning from the interrupt. The iret instruction pops the previously saved flags (including the interrupt flag which was originally enabled) and the return location so that the interrupted routine can continue processing none the wiser.