Ontology and User Assistance Architecture--
User assistance architecture has more to do with elements: how they are presented, organized, and behave--rather than the actual content within a specific instance. Ontology is the inventory, so to speak, of what elements you have at your disposal as an architect.

I am currently working on a wireframe for an embedded help pane and must decide what can go into or be accessed within that pane. The following is a sample ontology for such a pane:
Search: A way for the user to enter search criteria and initiate a search
Search Results: The list that the search returns
Links: Interactive text that navigates through content
Buttons: Command devices that initiate action
Headings: Elements that describe associated content
Multimedia: Elements such as graphics, e-learning, show-me demos
Documents: Large prewritten discourses such as user guides in PDF
Contacts: Tech support or other users who can help solve a problem
Knowledge Base: A database of known problems and recommendations
Information Blocks: Information displayed in small chunks. Blocks can contain several sub-types of information objects. It is useful to identify them, since you may want different types of information to be displayed differently. The following is a breakdown of possible information blocks:

Information Blocks
Definitions: What a term means
Purpose Statements: What a screen or module is intended to do
Guidelines: Higher order information a user needs to know in order to apply the screen or application within a user-goal context. For example, what are the impacts of enabling or disabling a feature or what should be considered when choosing among radio buttons A, B, or C.
Procedures: Sequence of steps to accomplish a task
Orientation: What impacts the current screen/task; what is impacted by the current screen/task

Food for Thought
As technical communicators, we are often drawn to procedural information as the core of user assistance. The sophistication of user interface design practices, however, often obviate this kind of information. See Procedures: the Sacred Cow Blocking the Road? . Consider this when prioritizing what types of information to present at the highest levels.