Category Associations do not directly reference another entity, and are used primarily to organize questions. Categories have a Display Name property, seen by the user, as well as a Canonical Name property. The canonical name is used to match and merge category instances. In some cases, multiple developers may need to independently define certain semi-common categories, but the questions should be sorted together on a running system.
Domains are like high-level categories. A single domain definition corresponds to a specific domain (for example, DB2), and it logically associates all questions and question viewers in that domain. It is also the basis by which activities are discovered by the Domain Viewer component.
In addition to the properties included with categories, a domain has an icon associated with it, that visually identifies the domain, and an Overview view which provides a default view of the domain. The definition also includes the Parents property which defines how the domain relates to other domains.
Messages inform the users that are currently logged into the browser interface of any activities that may affect them. This way you can quickly inform the end user when a background process that they started is finished. For example, when a process that prints a report in the background finishes, you can send a message to notify the user.
A message includes the message text and message details. It also contains a link to a relevant view, if its View property is set. For example, if you need to show a message that references a view displaying more information, if the View property of the Message Association is set, the view name appears as a link in the message. Clicking the link displays the specified view.
Customizable Associations form a special class of free-form associations that can express relationships between elements of their solution domain. Customizable Associations that are interchangeable have a common Group Tag property, that essentially identifies the purpose that a particular association fulfills. A Customizable Association has an icon that can be used to visually represent the association or its target, and a free-form Value property that is a list of the type Any. The association can refer to a single view, for instance, or a list of domain associations, or whatever entities or data you need to be associate with this instance.