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+0636 forms with similar adjacent characters prevents a line break inside it. The glyph can be confused with 28 other glyphs.
The Wikipedia has the following information about this codepoint:
Ḍād (ﺽ) 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, ẓāʾ, 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, ẓâ, ġayn, pe, che, že and gaf). In name and shape, it is a variant of ṣād. Its numerical value is 800 (see Abjad numerals).
In Modern Standard Arabic and many dialects, it represents an "emphatic" /d/, and it might be pronounced as a pharyngealized voiced alveolar stop , pharyngealized voiced dental stop [d̪ˤ] or velarized voiced dental stop [d̪ˠ]. The sound it represented at the time of the introduction of the Arabic alphabet is somewhat uncertain (most likely around late seventh to early eighth centuries), likely a pharyngealized voiced alveolar lateral fricative or a similar affricated sound [d͡ɮˤ] or [dˡˤ]. One of the important aspects in some Tihama dialects is the preservation of the emphatic lateral fricative sound [ɮˤ], this sound is likely to be very similar to the original realization of ḍād, but this sound ([ɮˤ]) and [ðˤ] are used as two allophones for the two sounds ḍād ض and ẓāʾ ظ.