US Mesh and Micron are the measurement units especially for filter or strainer maximum allowable particle arrest size based on no of pores in a unit area
US Mesh can also be written in inches or microns based on no of pores in the strainer , i will give you an example:
30 US Mesh = 595 microns
120 US Mesh= 125 microns
400 US mesh = 37 microns
More the mesh, smaller the micron size . There is a table for conversion , but i don't have it readily available, i just pulled out the figures from a fuel oil system engineering exercise , which i did recently.