You can add the minimum stock level value on products, in the “Properties” folder under 'Main Data' on ART. 
Here you will find the Reorder point, and if needed - the Reorder quantity. 
When this is filled in, you will get a list in the Purchase Handling application (PUR) when changing the MODE to ‘MinStock’.

Nb! A batch job runs ca every 5 minutes updating these values so you might notice a slight delay before the article appears in the purchase list (PUR)