From Newsgroup: comp.databases.ingres

U+03BC is Greek alphabet letter - Greek Small Letter Mu
U+00B5 is a symbol - Micro sign
If you check the Unicode Script section at https://www.compart.com/en/unicode/U+03BC https://www.compart.com/en/unicode/U+00B5
the former resides in "Greek" script,
the latter in "Code for undetermined" script.
As such the latter is not considered part of the Greek language/alphabet but a mere symbol,
"unit prefix in the metric system denoting a factor of 10reA6 "
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