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+0688 forms with similar adjacent characters prevents a line break inside it. The glyph can be confused with one other glyph.
The Wikipedia has the following information about this codepoint:
Ḍal or ḍāl is a letter of the extended Arabic alphabet, derived from dāl (د) by placing a small t̤oʾe (ط; historically four dots in a square pattern, e.g. ڐ) on top. It is not used in the Arabic alphabet itself, but is used to represent a voiced retroflex plosive [ɖ] in Urdu, Punjabi written in the Shahmukhi script, and Kashmiri as well as Balochi. The small t̤oʾe diacritic is used to indicate a retroflex consonant in Urdu. It is the twelfth letter of the Urdu alphabet. Its Abjad value is considered to be 4. In Urdu, this letter may also be called dāl-e-musaqqalā ("heavy dal") or dāl-e-hindiyā ("Indian dal"). In Devanagari, this consonant is rendered using ‘ड’.
Representations
System
Representation (click value to copy)
Nº
1672
UTF-8
DA 88
UTF-16
06 88
UTF-32
00 00 06 88
URL-Quoted
%DA%88
HTML hex reference
ڈ
Wrong windows-1252 Mojibake
Úˆ
Encoding: CP1256 (hex bytes)
8F
Encoding: GB18030 (hex bytes)
81 31 8D 36
Adobe Glyph List
afii57512
Adobe Glyph List
ddalarabic
RFC 5137
\u'0688'
Bash and Zsh inside echo -e
\u0688
C and C++
\u0688
C#
\u0688
CSS
\000688
Excel
=UNICHAR(1672)
Go
\u0688
JavaScript
\u0688
Modern JavaScript since ES6
\u{688}
JSON
\u0688
Java
\u0688
Lua
\u{688}
Matlab
char(1672)
Perl
"\x{688}"
PHP
\u{688}
PostgreSQL
U&'\0688'
PowerShell
`u{688}
Python
\u0688
Ruby
\u{688}
Rust
\u{688}
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