The values of these properties are used to populate user interface controls on the form. The system determines which form to launch by looking at message class of the message. Thus, the concept is really straightforward. MAPI forms are stored as messages in MAPI folders. Having this entry identifier you could open a message as you would do for a normal message and examine its properties.
Standard content holds standard items such as messages.
Associated content holds associated items such as forms, views, rule templates, reply templates and others. Associated content is usually hidden from users. A form definition does not actually define user interface for a form. Instead, it defines properties that go with the form. Also, these executables are saved as attachments to the form definition message.