According to the historical origin of the derivational elements.
No word is ever made a preposition by the addition of a derivational element.
In Nootka we have an unusually large number of derivational affixes (expressing concepts of group II).
In fishery, as compared with fish, the number of derivational elements is two.
At present it is a derivational syllable by which we can convert an adjective into an adverb: brave, brave-ly.