In fact Partitive Case in Russian exists for all uncountable nouns, but its form mostly coincides with Genitive and therefore it is not listed among standard Russian cases.

It can be used for uncountable X instead of a phrase "some X" (or sometimes "a plenty of X" or "a <somethingfull> of X") where otherwise X should be in Accusative.

For example:
Дай мне хлеб. = Give me a piece of bread. хлеб is in Accusative.
Дай мне хлеба. = Give me some bread. хлеб is in Partitive.

For some words Partitive has an alternative form which does not coincide with Genitive (ending -у instead of -а for first declension words) and that is why it is still a separate case rather than one more function of Genitive.


Свари супа.
Свари супу.
= Cook some soup.

Снегу намело!
Снега намело!

= What a plenty of snow is brought by the wind!