The partitive case is a grammatical case which denotes "partialness",
"without result", or "without specific identity".
(http://languagelink.let.uu.nl/tds/onto/LinguisticOntology.owl#partitiveCase
with reference to http://en.wikipedia.org/wiki/Partitive)
PartitiveCase expresses the partial nature of the referent of the
noun it marks, as opposed to expressing the whole unit or class
of which the referent is a part. This case may be found in items
such as the following: existential clauses, nouns that are
accompanied by numerals or units of measure, or predications of
material from which something is made. It often has a meaning
similar to the English word 'some' (Pei and Gaynor 1954: 161;
Richards, Platt, and Weber 1985: 208; Quirk, et al. 1985: 249;
Gove, et al. 1966: 1648; Sebeok 1946: 1214).
(http://purl.org/linguistics/gold/Partitive)