Question: 

The T6-600, T6-1000 and T6-1000 PRO are "open fork" clampmeters, but what is actually the difference ? 

Answer:

Fluke T6-600, T6-1000:

This test tools provide a touch electrode on the backside of the instrument, which must be used as earth reference.
Optionally the user could use the black test lead for external earth connection to get better accuracy.

Fluke T6-1000 PRO:

This test tool is designed for industrial electricians working in potentially dangerous environments.  
The T6-1000 PRO does not provide a touch electrode on the backside of the instrument, and is designed to always use an external ground clip.  
Since industrial electricians will almost always be wearing gloves or other PPE, a touch electrode is not very helpful.

In addition, the area where FieldSense technology measures voltage is extended in the open fork area at the T6-1000 PRO.
This means getting more consistent, accurate voltage and current measurements without touching a live wire is easier than ever before.

Also a new feature is added "Visual Continuity" for improved continuity testing.
Below a continuity value of approx 75 Ohm the LCD backlight will light up green.

Please watch this video as well:  https://www.youtube.com/watch?v=tgbkL2LLWZs