This method creates a uniquely-named form definition file and returns a FormDefinition object which represents that file. Because this is a virtual function, defined by the FormSet class, the form is returned as a FormDefinition object. In reality, the object is a FormDefinitionFile object, which is derived from FormDefinition.
The name of the form definition file will begin with the value of the NewFormFilenamePrefix property, followed by a unique number, followed by the value of the NewFormFilenameSuffix. The descriptions of those properties contain additional details.
Note that it is not necessary to call this method to create a new FormDefinitionFile object. It is also possible and acceptable to directly create a FormDefinitionFile object and set the Filename property to any value you want. However, using this method is easier and is more flexible if you choose to derive your own classes from FormSet and FormDefinition in the future.