This character is a Nonspacing Mark and is mainly used in the Khmer script.
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+17BC offers a line break opportunity at its position depending on the further context.
The Wikipedia has the following information about this codepoint:
Khmer script (Khmer: អក្សរខ្មែរ, Âksâr Khmêr[ʔaksɑːkʰmae]) is an abugida (alphasyllabary) script used to write the Khmer language, the official language of Cambodia. It is also used to write Pali in the Buddhist liturgy of Cambodia and Thailand.
Khmer is written from left to right. Words within the same sentence or phrase are generally run together with no spaces between them. Consonant clusters within a word are "stacked", with the second (and occasionally third) consonant being written in reduced form under the main consonant. Originally there were 35 consonant characters, but modern Khmer uses only 33. Each character represents a consonant sound together with an inherent vowel, either â or ô; in many cases, in the absence of another vowel mark, the inherent vowel is to be pronounced after the consonant.
There are some independent vowel characters, but vowel sounds are more commonly represented as dependent vowels, additional marks accompanying a consonant character, and indicating what vowel sound is to be pronounced after that consonant (or consonant cluster). Most dependent vowels have two different pronunciations, depending in most cases on the inherent vowel of the consonant to which they are added. There are also a number of diacritics used to indicate further modifications in pronunciation. The script also includes its own numerals and punctuation marks.