U+0342 Combining Greek Perispomeni

The Wikipedia has the following information about this codepoint:

Greek orthography has used a variety of diacritics starting in the Hellenistic period. The more complex polytonic orthography (Greek: πολυτονικό σύστημα γραφής, romanized: polytonikó sýstīma grafī́s), which includes five diacritics, notates Ancient Greek phonology. The simpler monotonic orthography (Greek: μονοτονικό σύστημα γραφής, romanized: monotonikó sýstīma grafī́s), introduced in 1982, corresponds to Modern Greek phonology, and requires only two diacritics.

Polytonic orthography (from Ancient Greek πολύς (polýs) 'much, many', and τόνος (tónos) 'accent') is the standard system for Ancient Greek and Medieval Greek. The acute accent (´), the circumflex (ˆ), and the grave accent (`) indicate different kinds of pitch accent. The rough breathing (῾) indicates the presence of the /h/ sound before a letter, while the smooth breathing (᾿) indicates the absence of /h/.

Since in Modern Greek the pitch accent has been replaced by a dynamic accent (stress), and /h/ was lost, most polytonic diacritics have no phonetic significance, and merely reveal the underlying Ancient Greek etymology.

Monotonic orthography (from Ancient Greek μόνος (mónos) 'single', and τόνος (tónos) 'accent') is the standard system for Modern Greek. It retains two diacritics: a single accent or tonos (΄) that indicates stress, and the diaeresis (¨), which usually indicates a hiatus but occasionally indicates a diphthong: compare modern Greek παϊδάκια (/paiðakia/, "lamb chops"), with a diphthong, and παιδάκια (/peˈðakia/, "little children") with a simple vowel. A tonos and a diaeresis can be combined on a single vowel to indicate a stressed vowel after a hiatus, as in the verb ταΐζω (/taˈizo/, "I feed").

Although it is not a diacritic, the hypodiastole (comma) has in a similar way the function of a sound-changing diacritic in a handful of Greek words, principally distinguishing ό,τι (ó,ti, "whatever") from ότι (óti, "that").