From ARK

"Items" in ARK are key database records to which other data is attached. They act as unique keys for series within the data such as context numbers or sample numbers. Items along with Fragments make up the key 'atomic' units of data in ARK. Items are series within Modules and are identified by a unique key value pair. Each item will therefore be identified by its Itemkey and Itemvalue. The Itemkey indicates which module the item belongs to and the Itemvalue is its unique identifier within that module.