There are differing opinions across the DoD of what is meant by interoperability. Some users consider the ability to translate data into text files and exchange them using simple e-mail as "achieving" interoperability. This is one way for two systems to work together, but this restricted view leaves out many other capabilities that are needed to satisfy an operational need. LISI expands the definition of interoperability beyond the ability to move data from one system to another -- it considers the ability to exchange and share services between systems. LISI focuses on increasing levels of sophistication for system-to-system interaction; i.e., thresholds of capabilities that systems exhibit as they improve their ability to interact with other systems. The specific capabilities needed to achieve each level are described in terms of four attributes – procedures, applications, infrastructure, and data, which are represented by the "PAID" acronym.