Hank,
It's the way things are changing, all the old engineering books like Chapman etc are totally out of date as regards tooling. For one thing what we knew as cutter descriptions have changed out of all context and even the manufacturers come up with their own descriptions.

At one time we had slot drills and end mills, slot drills are two flute with one flute going over centre so it can plunge and hey ho cut a slot.
Ends mills were 4 flute and had a hole in the centre for the grinding wheel to run out into and for that reason they couldn't plunge cut, you had to start off the work and move in hey ho hence 'end' of the work.

However modern CNC grinding techniques mean we can get 2, 3, 4, 5 or even 6 flute with or without centre cutting. Confused :question: well you will be.
The manufacturers haven't helped in that they call some cutters 4 flute end mills but they can plunge cut and on the same page they call a similar cutter a 4 flute slot drill.

So basically they have thrown the book out the window and it's a free for all. The old standards and terminology have gone forever. Don't believe me look in J&L book, or Cromwell, or WNT

Now the next bit is shanks, we had 4 sizes in metric and imperial.
6,10, 12 and 16 or 1/4", 3/8", 1/2" and 5/8" with 1/8" or 3mm for the tiny sub spindles. Many were threaded to fit the Clarkson type chucks.

With the advent of Carbide it was harder to grind the thread and the Clarkson fell out of favour for the straight collet chucks such as the ER series.
Because the ER series has no gaps in the range unlike a 1/2" Clarkson which can only hold 1/2" it then became viable to make the cutter size out of the blank size so we get things like a 7mm cutter ground from a piece of 7mm stock, add to this it can have 2, 3 or 4 flutes and still be centre cutting and you can see how we have moved on.

Hope that answers your question.

.