Data semantics is an well recognized issue in areas of information systems research. It provides indispensable information for management of data, It describes what data mean, how they are created, where they can be applied to, to name a few. Because of these diverse nature of data semantics, it has been described from different perspectives of formalization. This article proposes to formalize data semantics by the processes that data are created or transformed, A scheme is proposed to describe the structure that data are created and transformed, which is called Bill of Data. Bill of Data is a directed graph, whose leaves are primary input data and whose internal nodes are output data objects produced from input data objects. Using Bill of Data, algorithms are developed to compare data semantics.