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 Glyph for U+0066Latin Small Letter F, Glyph for U+006DLatin 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+3399 offers a line break opportunity at its position, except in some numeric contexts.
The Wikipedia has the following information about this codepoint:
The femtometre (American spelling femtometer), symbol fm, (derived from the Danish and Norwegian word femten 'fifteen', Ancient Greek: μέτρον, romanized: metrοn, lit. 'unit of measurement') is a unit of length in the International System of Units (SI) equal to 10−15 metres, which means a quadrillionth of one metre. This distance is sometimes called a fermi and was so named in honour of Italian naturalized to American physicist Enrico Fermi, as it is a typical length-scale of nuclear physics.
Representations
System
Representation (click value to copy)
Nº
13209
UTF-8
E3 8E 99
UTF-16
33 99
UTF-32
00 00 33 99
URL-Quoted
%E3%8E%99
HTML hex reference
㎙
Wrong windows-1252 Mojibake
㎙
Encoding: CP949 (hex bytes)
A7 AB
Encoding: EUC_KR (hex bytes)
A7 AB
Encoding: GB18030 (hex bytes)
81 39 E5 35
Encoding: ISO2022_JP_2 (hex bytes)
1B 24 28 43 27 2B 1B 28 42
Encoding: ISO2022_KR (hex bytes)
1B 24 29 43 0E 27 2B 0F
Encoding: JOHAB (hex bytes)
DC 3B
Adobe Glyph List
fmsquare
RFC 5137
\u'3399'
Bash and Zsh inside echo -e
\u3399
C and C++
\u3399
C#
\u3399
CSS
\003399
Excel
=UNICHAR(13209)
Go
\u3399
JavaScript
\u3399
Modern JavaScript since ES6
\u{3399}
JSON
\u3399
Java
\u3399
Lua
\u{3399}
Matlab
char(13209)
Perl
"\x{3399}"
PHP
\u{3399}
PostgreSQL
U&'\3399'
PowerShell
`u{3399}
Python
\u3399
Ruby
\u{3399}
Rust
\u{3399}
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