The major activity in a warehouse is material handling, which is focused on the input and output of stored goods. The purpose of such a warehouse is to guarantee a certain service to its clients. This can only be achieved if the clients' conditions to deliver the asked products at the right time can be met. The functions of a warehouse can be divided into storage, transhipment, the co-ordination of transport, logistical service, the grouping of products and a co-ordination of the co-operation between several distribution centres.

Not only visually, but also functionally there are a number of distinctions between warehouses. The most important distinction in functionality can be made between a production warehouse and a distribution warehouse.
A warehouse where the products only are stored (temporarily), is commonly seen at factories and is of secondary importance to the production process. The produced goods are stored in the warehouse until they are send to a buyer. A trading warehouse or a distribution centre could be such a buyer. In a distribution centre different products are received from an external supplier, stored and send to the buyer. No value is added to the products itselves, but by storing (possibly in smaller quantities) and regrouping them, the distribution of the products can be much cheaper than a direct delivery from producer to client. In many warehouses, one or more of the following storage systems can be found: a wide aisle pallet warehouse, a narrow aisle pallet warehouse or a shelf area warehouse. It is also possible to have a combination of these categories, so that besides a pallet warehouse a shelf warehouse is used.

Each of these three storage strategies is described below.

A wide aisle pallet warehouse


A wide aisle pallet warehouse is a warehouse with aisles, which usually have a width between 2.1 and 3.5 metres. The items are stored on pallets usually because the items or their number do not fit on shelves. The top rack storage levels are used for bulk stock and the lower levels for pick storage.
The order pickers use some kind of truck to collect the items. Several trucks are suited for picking items in such a warehouse; material handling equipment can consist of counterbalanced trucks or reach trucks for full pallet handling and pallet trucks, orderpicking carts or trucks for picking the items. These trucks can easily change aisles and can collect items from all over the warehouse as opposed to aisle bound Automatic Storage and Retrieval Systems (AS/RS). There is no extra time needed to change aisles and it is common that the aisles can be traversed in both directions. The height of the pallet racks vary, but the average might be around 7 metres high.
Examples of such warehouses can be found in distribution centres in the food industry.

A narrow aisle pallet warehouse


This kind of warehouse is often used for the storage of fairly large, not fast moving, items that are stored in pallet areas. A narrow aisle pallet warehouse is a warehouse with aisles, which usually have a width between the 1.5 and 1.8 metres. The products are stored on pallets because the items or the storage quantity do not fit on shelves.
Material handling equipment can exist of orderpicking cranes or narrow aisle highbay trucks. In these warehouses, the pallet racks are much higher than in wide aisle pallet warehouses (usually over 10 metres high), that is why the cranes or trucks have to reach higher and thus are heavier. These trucks cannot easily change aisles; the extra time, needed to change aisles is about 30 seconds. The aisles can be traversed in both directions.

A shelf warehouse


A shelf warehouse is a warehouse with aisles, which usually have a width between the 1.2 and 1.5 metres. The items are stored on shelves and usually are ordered in small quantities.
In certain shelf warehouses it is possible to walk through the aisles while picking the ordered items, because the distance between them is not very large. In other warehouses a little cart is used to collect   the items. There is no extra time needed to change aisles. The items have to be reached from ground position so that the top shelves cannot be higher than 2 metres.