This video has a big SHIS code system that I have not fully digested. These measurements would be used if you were upgrading the headset.

Video starts with cut away diagram
 - Internally routed: he goes thru a headset with internal cable
 Tedious to service. Not something you want on our standard utility bike.
 3:50 Two basic types 1. Threadless and 2. Threaded

 5:42 Park Tools SHIS system to identify headset type
 
 EC-External Cup (Brodie)
 ZS Zero Stack (Opus FSA headset)
 IS Integrated System

Our two bikes with unthreaded forks, I'm not sure. Chat: Brodie steel frame mountain bike is probably EC34/28.6

Then he talks about different diameters of steerer for both threaded and threadless.

 - stack height