The dependent vowel [o] (a-series), [u] (o-series).
The "conversion", which is a vertical mark written under a consonant, used in place of the diacritics ត្រីសព្ទ (◌៊) and មូសិកទន្ត (◌៉) if they clash with superscript vowels. It is identical to the dependent vowel ុ ([o], [u])