Sentence:  This week we take a close look at four top contenders in the multitasking arena ... All the multitasking systems available for microcomputers today fall into one of two distinct categories: "<WSD>bolt-on</WSD>" multitaskers, which add multitasking to an operating system (DOS, for example) that originally executed one task at a time; and "built-in" multitaskers, which are part of the operating system by design. 
Meaning of the word marked with <WSD> tags:  Used to extend an existing system, operating as an add-on or plug-in.