This character is a Other Symbol and is commonly used, that is, in no specific script.
The glyph is a square composition of the glyphs Greek Small Letter Mu, Latin Small Letter M. Its East Asian Width is wide. In bidirectional text it is written from left to right. When changing direction it is not mirrored. U+339B offers a line break opportunity at its position, except in some numeric contexts.
The Wikipedia has the following information about this codepoint:
The micrometre (Commonwealth English) as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System of Units (SI) equalling 1×10−6 metre (SI standard prefix "micro-" = 10−6); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.00004 inch).
The nearest smaller common SI unit is the nanometre, equivalent to one thousandth of a micrometre, one millionth of a millimetre or one billionth of a metre (0.000000001 m).
The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria, and for grading wool by the diameter of the fibres. The width of a single human hair ranges from approximately 20 to 200 μm.