add-on vs fitment

add-on

noun
  • Something which can be appended to something else. 

  • A software extension or hardware peripheral that provides additional functions or customization for a core application or system. 

  • Additional chips that can be purchased at specific times in tournament play. 

fitment

noun
  • A thing fitted to another in order to accomplish a specific purpose. 

  • The categorisation of an employee, for the purpose of calculating salary or allowances. 

  • An item of permanent furniture or equipment. 

  • The proper positioning and orientation of a thing for it to serve its designed purpose. 

  • The act of furnishing with fitments; an instance of such an act. 

How often have the words add-on and fitment occurred in a corpus of books? (source: Google Ngram Viewer )