This meter, often 0-30mA is used to read the real output power of your laser tube.
The power of a standard K40 is roughly 30-32 watts and the laser tube operates up to 16.000 volts (roughly).
Keeping the value below 16mA is a good precaution to save the life span of your laser tube. The output power can go up to 18-20mA but you run a high risk of over powering your tube and killing it quicker than running it on a lower power output.
If you K40 does not have a mA-meter installed, it´s a good and cheap solution to install one – click here for installation guide.