This is a possible parameter for ATOMIC_BLOCK. When used, it will cause the ATOMIC_BLOCK to force the state of the SREG register on exit, enabling the Global Interrupt Status flag bit. This saves on flash space as the previous value of the SREG register does not need to be saved at the start of the block.
Care should be taken that ATOMIC_FORCEON is only used when it is known that interrupts are enabled before the block's execution or when the side effects of enabling global interrupts at the block's completion are known and understood.