It's under the definition of "Flight Duty Period", last sentence:

Examples of tasks that are part of the flight duty period include deadhead transportation, training conducted in an aircraft or flight simulator, and airport/standby reserve, if the above tasks occur before a flight segment or between flight segments without an intervening required rest period.


FAR 117.25:
(g) If a flightcrew member engaged in deadhead transportation exceeds the applicable flight duty period in Table B of this part, the flightcrew member must be given a rest period equal to the length of the deadhead transportation but not less than the required rest in paragraph (e) [10hrs w/ 8hr sleep opportunity] of this section before beginning a flight duty period.

I don't read that as tacking on the DH hours to the required rest, just requiring a rest period that is either 10 hrs or the length of the DH, whichever is longer.

That's a tough one and if interpreted literally makes no sense. In the guide it specifically states that if the entire FDP was a deadhead that was 11 hours (let's just say your max duty day was 10 hours) then you would need 11 hours of rest. But let's say you had an FDP that involved 9 hours of actual flying plus a dead head that was tacked on to the FDP sequence totaling 12 hours of actual FDP. Your deadhead exceeded the FDP by 2 hours but total dead head time was only 3 hours. Do you get 12 hours of rest or 10? Isn't 9 hours of flying plus a 3 hour deadhead a lot more tiring than 12 hours of deadheading?

I would think it would be up to you to make sure you don't exceed any limits. The company should keep track for you, but I don't think they're compelled to.

Both the pilot and the company are responsible for ensuring compliance. They both can get sanctioned.