It's about the declension of nouns in Polish language. In polish the nouns are variable not only in the terms of the number, the gender etc. but also in the terms of their function ( it's called the gramatical case). There are seven grammatical cases in polish. 1.dopełniacz (kogo? czego?) 2.celownik (komu? czemu?) 3.biernik (kogo? co?) 4.narzędnik ((z) kim? (z) czym?) 5.miejscownik (o kim? o czym?) 6.wołacz ( o! ). In your question the non-animated nouns which ends with an -a are in the gramatical case of dopłeniacz. In dopełniacz the non-animated masculin nouns ends with an -a or with an -u , and non-animated feminin nouns ends with an -y and -i. However, there is no gamatical rule which indicates whether to put an -a or an -u at the end of the noun.