135 legs = Only 135 pilots (SIC too if ops spec says yes) may log stuff (PIC and SIC as appropriate)

91 legs = Sole Manipulator w/ Proper FAA Pilot License may "LOG" PIC time (Regardless if he/she is actually the PIC listed by the 135 operator) however, no one may log SIC time (Not required = no logging)

*If during the PART 91 LEG, the pilot hands the controls to that right seat guy, he becomes the sole manipulator and therefore can log PIC time

**The pilot is NOT ALLOWED to hand the controls over on the 135 legs unless that person is... (already mentioned above)