3 columns
IFR - Any flight which is launched on an IFR flight plan. This is a joke column which I keep only in my computer logbook. I don't keep this in my hardcopy as it's useless.
Actual Instrument - Any portion of flight with sole reference to instruments AND on an IFR flight plan. It stands to reason, that if you are not on an IFR flight plan you should be on a VFR flight plan. If on a VFR flight plan you shouldn't in theory be doing any flight with sole reference to instruments. However....
I say this because where I fly we do a lot of flight over water with restricted visibiliy or at night, but ATC still call us VFR or SVFR. While I know that I am flying 'solely by instruments' due to no horizon or lights, the authorities won't allow us to 'log' this as IFR for the reasoning above.
Simulated Instrument - Any flight where view-limiting device is worn and flight is by sole reference to instruments.