While the rest of the world is happily using the metric system, the United States is holding on strong to the use of the British Imperial System for weights and measures
While we thankfully no longer measure things in units of barleycorn (yes, really, its equal to 1/3rd of an inch), the United States still uses measurements like inch, foot, yard, etc.
Your caliper may read in point (1/1000th of an inch), GSM (grams per square meter), or both.