What does unit value mean in GWP PrintBorder Print Size Definition?

In GWP Print Border library, there are two Print Size Definition files. one is PaperSizeDefinitionISO.xml, one is PaperSizeDefinitionANSI.xml.

The unit code in ISO is 4 and ANSI is 6.

Where to check the unit value?

for example, 4 equal to meter and 6 equal to inch? and what's other values equal to?