In a large company it is a department with its own software and meetings. In a small store it comes down to three numbers per product, checked every week: the pace it sells at, the lead time of its supplier, and the safety stock held against both going wrong. The plan is then a date and a quantity for each product's next order.
It pays most where lead times are long and the catalogue has grown past what one person can keep in their head, because a mistake on a sixty-day supplier is two months of either empty shelves or idle cash. Seasonal stores feel it most, since next season's buy has to be placed on the strength of the last one.
nouz does the arithmetic of demand planning without being a purchasing system. The reorder planner on Insights, Inventory dates and sizes every variant's next order from its own pace, read from its recent sales and, with a year of history, last year's same weeks, from the lead time set on the Products page and from the stock already on its way. The Forecast page adds the demand of the whole store, orders and net revenue month by month, which is what next season's buying has to serve. Raising purchase orders, tracking deliveries and talking to suppliers stay in your own tools.