U+0345 COMBINING GREEK YPOGEGRAMMENI

The Wikipedia has the following information about this codepoint:

The iota subscript is a diacritic mark in the Greek alphabet shaped like a small vertical stroke or miniature iota ⟨ι⟩ placed below the letter. It can occur with the vowel letters eta ⟨η⟩, omega ⟨ω⟩, and alpha ⟨α⟩. It represents the former presence of an [i] offglide after the vowel, forming a so‐called "long diphthong". Such diphthongs (i.e., ηι, ωι, ᾱι)—phonologically distinct from the corresponding normal or "short" diphthongs (i.e., ει, οι, ᾰι )—were a feature of ancient Greek in the pre-classical and classical eras.

The offglide was gradually lost in pronunciation, a process that started already during the classical period and continued during the Hellenistic period, with the result that, from approximately the 1st century BC onwards, the former long diphthongs were no longer distinguished in pronunciation from the simple long vowels (long monophthongs) η, ω, ᾱ respectively.

During the Roman and Byzantine eras, the iota, now mute, was sometimes still written as a normal letter but was often simply left out. The iota subscript was invented by Byzantine philologists in the 12th century AD as an editorial symbol marking the places where such spelling variation occurred.

The alternative practice, of writing the mute iota not under, but next to the preceding vowel, is known as iota adscript. In mixed-case environments, it is represented either as a slightly reduced iota (smaller than regular lowercase iota), or as a full-sized lowercase iota. In the latter case, it can be recognized as iota adscript by the fact that it never carries any diacritics (breathing marks, accents).

In uppercase-only environments, it is represented again either as slightly reduced iota (smaller than regular lowercase iota), or as a full-sized uppercase Iota. In digital environments, and for linguistic reasons also in all other environments, the representation as a slightly reduced iota is recommended. There are Unicode codepoints for all Greek vowels with iota adscript (for example, U+1FBC ᾼ GREEK CAPITAL LETTER ALPHA WITH PROSGEGRAMMENI), allowing for easy implementation of that recommendation in digital environments.