A data model is an expression of how a system is intended to be used, and a statement of how it should interact with other systems. As part of the development of the latest version of Preservica, the underlying data model was significantly altered, informed by what went before and by reference to the experiences and best practices distilled into the PREMIS Data Dictionary. This paper reports on some of the key decisions made in the application of the PREMIS concepts to an extant digital preservation system.