Most Russian nouns have six case forms, but some have one or two more. These additional cases are called Partitive and Locative.
The meaning of the Partitive is 'some'; nouns that lack a Partitive case form use the Genitive to express this meaning.
The meaning of the Locative is 'in/at/on' after the prepositions в/на; nouns that lack a Locative case form use the Prepositional to express this meaning with these prepositions.