Will leave the question as to why the button was named "Map Custom Fields"
to someone else :-) but no version of Outlook that I'm aware of has ever
provided the capability to export (or import) user-defined fields or use any
"custom field" defined on a custom form.

All that happens on the screen you're referring to is the selection process
of the fields you want to export but here is some thing to make note of:

If you have never completed an import/export function before - the right
hand side of the screen will be blank otherwise Outlook will remember the
fields selected from the last import/export function you completed. If
that's the case, then you simply need click on "Clear Map" and proceed with
selecting your fields.

Outlook will remember the last import/export activity that occured but it
does not provide the capability to create an import or export template for
future use. Ergo, if you only do ONE thing all the time, you could just go
ahead and run the action repeatedly without any "editing or mapping". For
some people, that's all they need. However, if anything different happens in
between you'll have to redo the map selection.

Karl