SiSU markup is fairly minimalistic, it consists of: a (largely optional) document header, made up of information about the document (such as when it was published, who authored it, and granting what rights) and any processing instructions; and markup within the substantive text of the document, which is related to document structure and typeface. SiSU must be able to discern the structure of a document, (text headings and their levels in relation to each other), either from information provided in the document header or from markup within the text (or from a combination of both). Processing is done against an abstraction of the document comprising of information on the document's structure and its objects, which the program serializes (providing the object numbers) and which are assigned hash sum values based on their content. This abstraction of information about document structure, objects, (and hash sums), provides considerable flexibility in representing documents different ways and for different purposes (e.g. search, document layout, publishing, content certification, concordance etc.), and makes it possible to take advantage of some of the strengths of established ways of representing documents, (or indeed to create new ones).