External staff is an umbrella term for individuals performing services for Microsoft on a non-permanent basis. Examples include consultants, temporary contract workers, vendor workers, freelancers, independent professionals and contractors, staff augmentation, and business guests.
Nothing in that language excludes cafeteria workers, janitors, HVAC repairmen, etc. Does MS really mean to restrict blue-collar workers to 18-month stints too? Their employers won’t necessarily have another gig available for them, and they’re far less likely than coders and managers to have a financial cushion.