A representation is a set of metadata describing the actual file containing document data. For example, the “declaration of independance” document, could have multiple representations, one in pdf, one in txt and so on. It is important thought to understand that representations of the same document must represent the same thing (exactly the same text, or the same picture at different resolution, or the same video encoded in different format). For example, a black and white version of a color video cannot be associated to the same document, a second document must be created for this. This rule is only here for consistency, the system do not enforce it. But a representation is picked depending of the context, and you don’t want iPhone users to end up with a black and white representation of a video while it should be in color.