Sentence:  The efforts and accomplishments fall into three main <WSD>categories</WSD> : absolute thermometry based upon the velocity of sound in helium gas , secondary thermometry involving principally studies of the behavior of germanium resistors , and helium 4 vapor-pressure measurements ( see p. 144 ) . 
Meaning of the word marked with <WSD> tags:  a general concept that marks divisions or coordinations in a conceptual scheme