It's feminine plural.
The singular is κοινοποίηση.
Two common noun forms from verbs are a feminine form in -ση and a neuter one in -μα. (Generally speaking the noun in -ση refers to the process of verbing and -μα to the result; for example, ποίηση is "poetry" (the process of creating) and ποίημα is "poem" (the created thing), both from ποιώ "to make, create".)
The -ση nouns have a slightly special conjugation with plural -σεις (as here) rather than -σες. Genitive plural is -σεων; genitive singular can be the traditional -σεως or a modernised/regularised -σης -- I believe the latter is more common.
Note also the movement of stress: κοινοΠΟΊηση, κοινΠΟΊησης - κοινοποιΉσεως, κοινοποιΉσεων, κοινοποιΉσεις.
Also one of those feminine plural nouns that had an -ις ending in Ancient Greek and so inflect slightly differently from "regular" -η feminine nouns.
Nearly all of them end in -ση (-ψη, -ξη) in the nominative singular, but a couple of them have a different consonant before the ending, e.g. δύναμη, πόλη.