Distribution systems for retail organizations are complex, whether they sell via brick-and mortar stores, online systems or combinations thereof (dual channels). The complexity stems from several sources, in particular:
(a) Retailers like Amazon deal with close to a billion distinct items;
(b) Inventories need to be kept at many locations: hundreds or thousands of stores ( Walmart) and close to a hundred fulfillment centers (Amazon).
(c) Retailers employ multiple sales channels and platforms (Amazon vs Amazon Marketplace)