Data book is one of the types of custom field in which values are selected from a drop-down list. The values are created in the adjacent "Data books" tab.
The screenshot above shows that there are several warehouses. If you have such a data book, you can create a custom field "Warehouse" and select the created data book for it.
Please note that when creating or editing a data book, there is a 1000 line limit on adding elements.
You can easily change the order of the values: drag them to the desired place using the mouse or use the adjacent arrows.