This character is a Nonspacing Mark and inherits its script property from the preceding character. It is also used in the scripts Arabic, Syriac.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it acts as Nonspacing Mark. When changing direction it is not mirrored. U+0651 prohibits a line break before it. The glyph can be confused with 9 other glyphs.
The Wikipedia has the following information about this codepoint:
Shaddah (Arabic: شَدّةshaddah[ˈʃæd.dæ], "[sign of] emphasis", also called by the verbal noun from the same root, tashdidتشديدtashdīd "emphasis") is one of the diacritics used with the Arabic alphabet, indicating a geminated consonant. It is functionally equivalent to writing a consonant twice in the orthographies of languages like Latin, Italian, Swedish, and Ancient Greek, and is thus rendered in Latin script in most schemes of Arabic transliteration, e.g. رُمّان = rummān 'pomegranates'.