Last year we wrote about the chip being part of the Health care bill see here
Well in the bill that passed it is right there. Whatever is meant with "implantable medical device".
(g)(1) The Secretary shall establish a national medical device registry (in this subsection referred to as the ‘registry’) to facilitate analysis of postmarket safety and outcomes data on each device that— ‘‘(A) is or has been used in or on a patient; ‘‘(B)and is— ‘‘(i) a class III device; or ‘‘(ii) a class II device that is implantable, life-supporting, or life-sustaining.”
In this old C-span video you will hear the voice of Joe Biden telling John Robersts Chief Justice that "Can a microscopic tag be implanted in a person’s body to track his every movement? There’s actual discussion about that..You will rule on that, mark my words before your 10 year? is over"