Its pretty straight forward (under the FARs of course).
Simulated instrument time in VMC can only be logged using a "view limiting device" so if flying on a nice sunny day and you decide to keep your head in the cockpit, you cannot log Simulated Instrument Time....and not forgetting the need for a safety pilot.
Its equally clear regarding Actual Instrument Time. If you are flying along in IMC and are flying with sole reference to the insturments (...and if you didn't, you'd die) then you may log actual instrument time.
Flight plans or flight rules don't come into it from a logging point of view. I have often flown IFR on an IFR flight plan on sunny days, but don't log actual instrument. Equally in the UK I have flown in accordance with IFR, in IMC on no flight plan and have logged Actual.
The AA 777 pilot flying from LAX to Heathrow will not log Actual for probably 99% of his flight, as he is not flying by sole reference to the instruments (he has a horizon....at least until the approach into Heathrow
)