Abstract: Based on the discussion of functional expansion of Finnish partitive marker, this paper proposes to distinguish mighty categories’ expansions between intra-domain and extra-domain functions. The Finnish partitive marker can be expanded to express unbounded quantity, indefinite, imperfective and negative meanings. The expansion to quantitative and referential categories reflects an intra-domain expansion and the expansion to aspectual and modal categories an extra-domain expansion. The extra-domain functional expansion is constrained by 1) the distance between the original and expanded categories, 2) the distance between the linear arrangements of language units, and 3) the limitations of the language inventory. As a result, the expansion faces difficulty in occurrence and low degree of cross-linguistic reproducibility; meanwhile, it plays a more important role in the shaping of the language inventory and language characteristics, which should be given attention to at the theoretical level.