This character is a Other Letter and is mainly used in the Palmyrene script.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it is written from right to left. When changing direction it is not mirrored. The word that U+10866 forms with similar adjacent characters prevents a line break inside it.
The Wikipedia has the following information about this codepoint:
The Palmyrene alphabet was a historical Semitic alphabet used to write Palmyrene Aramaic.
It was used between 100 BCE and 300 CE in Palmyra in the Syrian desert.
The oldest surviving Palmyrene inscription dates to 44 BCE.
The last surviving inscription dates to 274 CE, two years after Palmyra was sacked by Roman Emperor Aurelian, ending the Palmyrene Empire. Use of the Palmyrene language and script declined, being replaced with Greek and Latin.
The Palmyrene alphabet was derived from cursive versions of the Aramaic alphabet and shares many of its characteristics:
Twenty-two letters with only consonants represented
Written horizontally from right-to-left
Numbers written right-to-left using a non-decimal system
Palmyrene was normally written without spaces or punctuation between words and sentences (scriptio continua style).
Two forms of the Palmyrene alphabet were developed: The rounded, cursive form derived from the Aramaic alphabet and later a decorative, monumental form developed from the cursive Palmyrene.
Both the cursive and monumental forms commonly used orthographic ligatures.