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).
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: