In contrast, when information is stored into a CAS system, the system will record a , which is an identifier uniquely and permanently linked to the information content itself. A request to retrieve information from a CAS system must provide the content identifier, from which the system can determine the physical location of the data and retrieve it. Because the identifiers are based on content, any change to a data element will necessarily change its content address. In nearly all cases, a CAS device will not permit editing information once it has been stored. Whether it can be deleted is often controlled by a policy.