Once a contact folder has been selected for export, you can change the message class to export the contacts that are specifically assigned to that message class. For more detailed information - see Import <Selecting Message Class>

Ignore MessageClass
Unlike the import process where a specific message class must be selected, an additional option is available in the Export process - to ignore the message class and export all contacts in the folder.
The impact of selecting <Ignore MessageClass> is that no user-defined fields will be available for selection since the field will not be consistent for all contacts in the folder. Only standard fields can be selected for export using this choice.