A fixed$-$period $($FPS$)$ inventory system:

$$ orders a fixed$-$period quantity when the inventory position reaches or drops below the reorder point $(r).$

chooses a replenishment level $(M)$ which includes the demand during the review period and lead time, plus any safety stock.

maintains a constant order quantity every review period.

assumes that the demand for an item is constant and continuous, and that the items are withdrawn at a constant rate.