The meaning of the number varies by country/manufacturer. Higher numbers are always finer, but compare Testors 800 grit sanding film to 3M wet/dry, and and the difference is night and day. Problem is, manufacturers rarely tell you which scale they use (even in the US). The Euro standard has a "P" before the grit number, you may have seen this in a woodworking store.
And then there is Micromesh, which uses a different scale altogether.
Here are links to a couple of conversion charts. I find them quite helpful. Note that if you are using 1500 grit in the US ANSI standard, then switch to 1800 in Micromesh, you are actually getting a coarser grit, not finer.
https://www.fine-tools.com/G10019.html
http://www.sisweb.com/micromesh/conversion.htm