Sentence:  While the particular limits of these groupings may seem artificially arbitrary ; they do fairly express a corresponding grouping of more variable material , and they eventuate also in five <WSD>classes</WSD> , along a similar scale , containing approximately equal numbers of cases , namely 19 , 14 , 15 , 11 , 12 in Athabascan . 
Meaning of the word marked with <WSD> tags:  a collection of things sharing a common attribute