[Obi_Juan_Kenobi]

2.5 micron is basically the metric value for a 'tenth', i.e. one ten-thousandths of an inch. A typical machine shop will usually work in tolerances of a 'thou' or two, but most are equipped to measure to tenths without too much fuss. Standard micrometers have a vernier that reads tenths, and tenths indicators aren't uncommon.

Going beyond that, to tens of millionths or sub-micron, is where things get nutty with special climate controlled rooms. Digital indicators can easily read with such precision, but controlling for factors like radiated body heat become important.

Reference measurement is usually done with gauge blocks and pins. The development and popularization of these sets is basically how mechanical parts became standardized.

If you're interested in that sort of thing - bootstrapping precision and the like - check out "The Foundations of Mechanical Accuracy".