Although the TRAK.gstencil stencil is a single stencil file what you see / can access depends on what version of OmniGraffle you use:
Note: OmniGraffle allows you to use any block (shape) and any connector but only certain blocks and only certain connectors can be used for any particular TRAK View. The purpose of each TRAK view together with the content is specified in the TRAK Viewpoints document (http://trakviewpoints.sourceforge.net)
OmniGraffle Pro supports [User Data] and allows you to create and edit it. OmniGraffle and OmniGraffle for iPad do not allow you to see user data although you can see the results of it, for example, the type of each block is obtained from
<%UserData stereotype%>
and in each block there is an entry like
stereotype: System
which fills the block type label and keeps things consistent.
Although they do not allow you to edit user data they do not remove it (so you can move the diagram between different versions without losing information)
User data is provided for:
On the iPad just tap on the stencils button:
.
When you drag a shape off the stencil the type is automatically shown from the [User Data] within the shape.