After the order is placed, deterministic lead-time demand consumes stock before the receipt arrives.
Place order
At the reorder point, place an order for 12 units.
Lead-time demand
During lead time, deterministic demand consumes 6 units.
lead-time demand=6
Safety stock left
Safety stock left before receipt is 4 units.
Receipt
The receipt restores stock to 16 units.
4+12=16