For example, A21N using A321 base shows Aircraft: A321 in the first section of the output page.
Might be more confusing when creating a flight with a custom airframe where there is no IRL relation between the base type and specified type for the custom airframe.
On the other hand, if simBrief were to use e.g. the type designator then I suppose the page would mark the BBJ and BBJ2 as B737/B738 whereas the current setup allows for the distinction, I guess.
Regards,
Tim