It's not as unclear as you may think. Acting as PIC and Logging PIC are separate things.
Take the case of an instrument training flight on an IFR flight plan. The instructor is acting as PIC- he's in charge. The student can log PIC if he wishes because he's the sole manipulator.
Let's now look at an airline example. The guy in the left seat signed for the airplane, and is acting as PIC. When the FO is flying, he can log PIC as the sole manipulator.
In the airline environment though, the FO's next employer is unlikely to accept the time that was logged as PIC without acting as PIC. Clear?