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 Latin Small Letter M, 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+339C offers a line break opportunity at its position, except in some numeric contexts.
The Wikipedia has the following information about this codepoint:
The millimetre (international spelling; SI unit symbol mm) or millimeter (American spelling) is a unit of length in the International System of Units (SI), equal to one thousandth of a metre, which is the SI base unit of length. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre.
One millimetre is equal to 1000 micrometres or 1000000 nanometres.
Since an inch is officially defined as exactly 25.4 millimetres, a millimetre is equal to exactly 5⁄127 (≈ 0.03937) of an inch.
Representations
System
Representation (click value to copy)
Nº
13212
UTF-8
E3 8E 9C
UTF-16
33 9C
UTF-32
00 00 33 9C
URL-Quoted
%E3%8E%9C
HTML hex reference
㎜
Wrong windows-1252 Mojibake
㎜
Encoding: BIG5 (hex bytes)
A2 50
Encoding: BIG5HKSCS (hex bytes)
A2 50
Encoding: CP932 (hex bytes)
87 6F
Encoding: CP949 (hex bytes)
A7 AE
Encoding: CP950 (hex bytes)
A2 50
Encoding: EUC_JIS_2004 (hex bytes)
AD D0
Encoding: EUC_JISX0213 (hex bytes)
AD D0
Encoding: EUC_KR (hex bytes)
A7 AE
Encoding: GBK (hex bytes)
A9 4C
Encoding: GB18030 (hex bytes)
A9 4C
Encoding: ISO2022_JP_2 (hex bytes)
1B 24 28 43 27 2E 1B 28 42
Encoding: ISO2022_JP_2004 (hex bytes)
1B 24 28 51 2D 50 1B 28 42
Encoding: ISO2022_JP_3 (hex bytes)
1B 24 28 4F 2D 50 1B 28 42
Encoding: ISO2022_KR (hex bytes)
1B 24 29 43 0E 27 2E 0F
Encoding: JOHAB (hex bytes)
DC 3E
Encoding: SHIFT_JIS_2004 (hex bytes)
87 6F
Encoding: SHIFT_JISX0213 (hex bytes)
87 6F
Adobe Glyph List
squaremm
RFC 5137
\u'339C'
Bash and Zsh inside echo -e
\u339C
C and C++
\u339C
C#
\u339C
CSS
\00339C
Excel
=UNICHAR(13212)
Go
\u339C
JavaScript
\u339C
Modern JavaScript since ES6
\u{339c}
JSON
\u339C
Java
\u339C
Lua
\u{339C}
Matlab
char(13212)
Perl
"\x{339C}"
PHP
\u{339c}
PostgreSQL
U&'\339C'
PowerShell
`u{339C}
Python
\u339C
Ruby
\u{339c}
Rust
\u{339c}
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