This character is a Other Letter and is mainly used in the Arabic script.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it is written as Arabic letter from right to left. When changing direction it is not mirrored. The word that U+0638 forms with similar adjacent characters prevents a line break inside it. The glyph can be confused with 11 other glyphs.
The Wikipedia has the following information about this codepoint:
Ẓāʾ, or ḏ̣āʾ (ظ), is one of the six letters the Arabic alphabet added to the twenty-two inherited from the Phoenician alphabet (the others being ṯāʾ, ḫāʾ, ḏāl, ḍād, ġayn). It is also one of the ten letters the Persian alphabet added from the twenty-two inherited from the Phoenician alphabet (the others being s̱e, xe, ẕâl, zâd, ġayn, pe, che, že and gaf). In name and shape, it is a variant of ṭāʾ. Its numerical value is 900 (see Abjad numerals).
Ẓāʾظَاءْ does not change its shape depending on its position in the word:
Representations
System
Representation (click value to copy)
Nº
1592
UTF-8
D8 B8
UTF-16
06 38
UTF-32
00 00 06 38
URL-Quoted
%D8%B8
HTML hex reference
ظ
Wrong windows-1252 Mojibake
ظ
Encoding: CP720 (hex bytes)
E2
Encoding: CP1256 (hex bytes)
D9
Encoding: GB18030 (hex bytes)
81 31 85 36
Encoding: ISO8859_6 (hex bytes)
D8
Encoding: CP420 (hex bytes)
90
Adobe Glyph List
afii57432
Adobe Glyph List
zaharabic
digraph
zH
RFC 5137
\u'0638'
Bash and Zsh inside echo -e
\u0638
C and C++
\u0638
C#
\u0638
CSS
\000638
Excel
=UNICHAR(1592)
Go
\u0638
JavaScript
\u0638
Modern JavaScript since ES6
\u{638}
JSON
\u0638
Java
\u0638
Lua
\u{638}
Matlab
char(1592)
Perl
"\x{638}"
PHP
\u{638}
PostgreSQL
U&'\0638'
PowerShell
`u{638}
Python
\u0638
Ruby
\u{638}
Rust
\u{638}
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