Specify a name for the department. Departments can be anything that has meaning to you and helps you organize the way you move through the store. 'Bakery', 'Deli', and 'Produce' are all departments, but in some cases it may make sense to create a department that is named after an item, a store, or an aisle in a store.
Some items may be placed in different departments at different stores. For example, let's say that you buy bottled water at two stores. One store has bottled water with soft drinks and the other has bottled water with the non-carbonated drinks. In this case it may be helpful to simply create a department called Bottled Water and position that department appropriately in the two different stores.
You have the flexibility to get very detailed, or not, based on what works for you.