In the FAQ section of Navy.com is this question:
http://www.navy.com/faq.html
CAN I EXPLORE NEW FIELDS OUTSIDE OF THE JOB FOR WHICH I RECEIVE TRAINING?
Absolutely! Once you’ve served at least one tour in your designated field (Pilot, Naval Flight Officer, Intelligence, etc.) the opportunities open up. Here is just a sampling of the many opportunities available to Fleet-experienced Officers: Engineering Duty Officer, Public Affairs, Fleet Support, Recruiting, Space Command, Space and Electronic Warfare,
Joint and Foreign Service, and staff positions.
The OP posted; "I wanted to transfer to "Joint & Foreign Service" after a few years in the service but not sure being in Supply will let me do that because Supply Corps is listed under "restricted line".
My first response was the Foreign Service is a term that applies to a State Department Official
http://careers.state.gov/officer so it was not an option if he goes into the Supply Corps.
The OP responded that he read on the Navy FAQ page the term Joint and Foreign Service. At which point I responded that what the webpage means is overseas tours. So while the Navy used the term "Foreign Service" on their FAQ page, that term has a specific meaning which I interpreted as his desire to try to become a Foreign Service Officer.
The OP then posted that he may be interested in an Attache job and I responded that I haven't heard of Supply Corps Officers doing Attache jobs, all the ones I know were URL. The OP would need to do some research WRT to SuppO's doing an Embassy tour.
The only definition close to "foreign service" that I've found is:
JP 1-02 - Foreign service national. (DOD) Foreign nationals who provide clerical, administrative, technical, fiscal, and other support at foreign service posts abroad and are not citizens of the United States. The term includes third country nationals who are individuals employed by a United States mission abroad and are neither a citizen of the United States nor of the country to which assigned for duty. Also called FSN.
NTRP 1-02 does not have a specific definition for 'Foreign Service'.
That being said, I would say there is not formal definition for the term.