As said before its just the roof cap.
The model XF is the flagship for DAF trucks and there are 3 cab sises to chose from, depending the job the truck is doing.
1) The smaller is the sleeper cab (flat top), flat floor, one bed, mostly used for national or just local trips.
2) The midle cab Space cab (stand up), one or two beds, flat floor, more space and closets, mostly used for national or international trips.
3) The Super Space cab (stand up) (floor to roof more than 2 meters high), one or two beds, flat floor, more space, more closets, dedicated for international, or if you want many days trips. If I am correct the Super space cab was the biggest and cab in EU for many-many years.
You could classify cabs in proportion to those of Peterbilt as follows: DAF Sleeper - 36'' sleeper, DAF Space cab - 63'' stand up sleeper, DAF Super Space cab - 70'' sleeper.
Of course it all depents also to the owner operator and his pocket. For example, here in Greece there are many-many owners using the super space cab doing tranzit all over Europe and at the other hand there are others who hauls ship cans around, their biggest trip is up to 500klms and also use the same cab.
IMO, it is crasy to buy a truck with the biggest cab to do the job you can do with a day cab.
Hope that helps.
Kostas.