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.
El Wikipedia tiene la siguiente información acerca de este punto de código:
La ẓāʾ o ḏ̣āʾ (en árabe ﻇﺎء, ẓāʾ [ðˤaːʔ]) es la decimoséptima letra del alfabeto árabe. Representa un sonido fricativo, alveolar, sonoro y velarizado,[1] /ðˁ/ o /zˁ/. En la numeración abyad tiene generalmente el valor de 900, aunque en el Magreb suele tener el de 800.[2]