The "new kid" finds his own answer ! "What is a 'micron' ?"
 

A "micron" is typically a unit of length equal a micrometer or 1/1,000,000 of a meter or 1/1,000 of a millimeter (or 0.00003 inches or DAMN SMALL !).

When HVAC technician are taking about "microns" for a unit of absolute pressure, what they are referring to is a "microns of mercury" in a glass manometer. Atmospheric pressure is about 14.7 psi or 760 mm-Hg or 760,000 μm-Hg.

Another unit of extremely low pressure (near perfect vacuum) is a torr. 1 torr ~ 1,000 microns (μm-Hg).