All of your time in the aircraft that you fly for a certificate holder is "135 time."
There are times when you will not operate it in accordance with certain 135 requirements, but it will always be beholden to 135 maintenance standards, just as you'll always be beholden to currency under the certificate holder's training program, drug testing requirements, flight checking, etc.
When you're operating under Part 135, you're still operating under Part 91. The notion that you're either "Part 135" or "Part 91" is misplaced. You are better thinking of yourself operating under Part 91 at all times, with additional requirements (and relief) imposed by Part 135.
There is no FAA classification or definition of the logging of "Part 135" time.
You can complete your duty day, end your duty to that certificate holder, and fly the airplane on a repositioning flight as a "Part 91" flight; you're still an employee of the 135 certificate holder and you're still able to log that time as "135" time; doing so doesn't make it duty, nor does it impose any additional legal burden on you. All of your time operating the company aircraft may be assessed as "135 time," even if you're not on duty or operating under "135" at the time. If you're repositioning for the certificate holder after duty has concluded, you're still working for the certificate holder (even though as a flight after duty is over, the certificate holder can't compel you to make the flight).
Don't get too wrapped around the axle about Part 91 vs. Part 135. It's always Part 91, but sometimes has additional regulation to consider.