User Data is used to capture / store information and help keep things consistent. In doing so it is hoped that it provides a path if conversion into an exchange format such as XMI is needed.
User data keys can only be seen in OmniGraffle Pro. User data values can only be edited in OmniGraffle Pro.
The results of the data keys can, however, be displayed in any of the OmniGraffle applications - including iPad - if you know what the name of the user data key is:
<%UserData keyName%>
Blocks
Each block has user data keys that correspond to the attributes for that type in the TRAK metamodel (http://trakmetamodel.sourceforge.net)
Canvas
An OmniGraffle canvas has a number of attributes that are used in the view identification table:
<%Document%>
<Organizations%>
<%Authors%>
<%CreationDate%>
<%ModificationDate%>
<%version%>
<%Description%>