1. A computer-readable medium having stored thereon a data structure comprising: a first data field comprising information linking the data structure to a document; a second data field comprising information identifying a user-defined part of the document being marked by the data structure; and a third data field comprising information regarding user-defined properties of a visual indicator corresponding to the data structure, wherein the visual indicator is configured to be displayed partially outside of the document and with the user-defined part of the document.