Can one item have multiple item sites?

Yes, items can be located in multiple sites. For example, in a manufacturing environment, you might require an item to be located in a factory, a quality shop, and a storage warehouse. Distribution companies locate items in multiple warehouses, allowing them to transfer inventory between sites as the need arises. Keep in mind, though, you must be running a commercial edition of xTuple to have access to multiple sites. You can read more about the different xTuple features here.