I agree. Not much to interpret. A 135 certificate holder must designate an acting PIC for every flight. If the operation requires two pilots, the certificate holder must designate an acting SIC. And those designations do not change during the flight.

It's connection to the logging question is also pretty straightforward.

The designated PIC "acts as pilot in command of an aircraft for which more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is conducted" so she logs PIC under 61.51(e)(1)(iii) whether or not at the controls.

The designated SIC is acting as SIC on a flight where "more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is being conducted" so he logs SIC under 61.51(f)(2) whether or not at the controls.

And, of course, completely independent of either of those regs, whoever has the proper aircraft rating and is the sole manipulator of the controls gets to log his or her hands-on time as PIC under 61.51(e)(1)(i).