It depends on licensing authority. JAR 1.080 c) (5) explains
(5) PICUS (Pilot-in-command under supervision)
Provided that the method of supervision is acceptable to the Authority, a co-pilot may log as PIC flight time flown as PICUS, when all of the duties and functions of PIC on that flight were carried out, such that the intervention of the PIC in the interest of safety was not required.
For the UK CAA every flight with any captain is an acceptable method of supervision, it doesn't need to be a pilot flying sector either. For most other european authorities you can only log PICUS in a structured training environment, usually partly done during the initial license training and the rest during a command course.
As far as i know it will change again next year with the new european licensing rules anyway.