They're not really confusing if you understand paint. The problem is there are so many different paint manufacturers and they all have their own codes as the tinters they use are different.

The paint shop I found uses Hitex paint, fortunately when they colour matched the old Dulon paints they mostly kept the Dulon code and just put a D in front of the code for Dulon however their tinter codes and mixing amounts are different to the Dulon ones...

If you look at the chart above Gazelle low gloss and med gloss interior is listed with all the tinter codes and amounts to be added to make Gazelle, these are Dulon tinters (now PPG) and the Hitex paint tinter codes and amounts are different. Same applies to each paint manufacturer.