Sentence:  The author ... supplied elaborate rules for the use of the octave system and for the reducing of numbers from the decade to the octave system, and the reverse — processes which he called <WSD>octavation</WSD> and decimation, respectively. 
Meaning of the word marked with <WSD> tags:  (Conversion) (of the (expression) of a (number)) from (denary) to (octal) (notation).