I think what Jackie means is that the
amount of headspace is expressed in thousants of an inch (in the US where the inch measuring system is used).
The standard definition if headspace, regardless if it is a rimmed, belted or rimless cartridge, is "the distance between the face of the bolt and the base of the cartridge with the cartridge
fully seated in the chamber". If the firearm is not a bolt type action, say for example a revolver or semi-auto pistol, then the definition is "the distance between the face of the breech and the base of the cartridge with the cartridge fully seated in the chamber".
If you are buying a new custom or benchrest rifle or having a custom or benchrest rifle built the gunsmith can use the chambering reamer and a short section of the barrel to make a gage to aid in setting back the shoulder if headspace becomes negative, being determined by the bolt being hard to close when inserting resized fired brass that is still not primed or loaded, and using a stripped bolt.
There are aftermarket products like the one shown by Al Nyhus in his post on annealing brass
http://www.benchrest.com/forums/showthread.php?t=54166 .
The trouble I have with this type device where it has only a single contact point is that your chamber shoulder may not be the
exact same angle as the sizing die shoulder. That is why many experienced reloaders use the stripped bolt "feel" method.