Form and Alias handling
Notes and Domino use the value of the form item to determine what form to load when displaying a document. The first match of a form item value (or a form formula in a view or folder) with either a form name or any of the 0:n aliases determines what form is used to render a document.
This is in line with Model-View-Controller where the document has the role of the model and the form serves as the view (kind of).

REST API form handling
The REST API uses the information from the document's form item to determine access by looking at the configuration loaded from the active schema accessed through the scope.
The approach follows the same principle as the Notes client or the Domino HTTP task, just the lookup source is the schema and not the list of forms in the design.
Warning
The REST API normalizes the value of the form item. The returned form item corresponds to the last alias of the found form configuration. The stored document itself remains unchanged.

The rationale
In contemporary development understanding, there are label/value pairs. To normalize Notes documents, we follow this pattern, using the form name as the label and the last item as the value.
Any intermediate aliases are treated as variations and are excluded from API use to simplify access for external developers.