Normally, Dynamics AX forms are created in AOT by adding various controls to the form's design and do not change during runtime. But besides that, Dynamics AX allows developers to add controls dynamically during form runtime.
Probably, you have already noticed that the Document handling form in the standard Dynamics AX application has a nice option to create a new record by clicking the New button and selecting the desired document type from the list. This feature does not add any new functionality to the application, but it provides an alternative way of quickly creating a new record and it makes the form more user-friendly. The content of this button is actually generated dynamically during the initialization of the form and may vary depending on the document handling setup.
There might be other cases when such features can be used. For example, dynamic menu buttons could be used to display a list of statuses, which depends on the type of the selected record...