SNY = Supernumerary
i.e. You are not a required member of the crew, and as such you cannot log the time in your log book as a crew member. It merely serves as a means of recording flying experience in your book without it contributing to your totals.
As nearly all SEP aircraft specify the minimum crew as one pilot, your choices for the log book are:
P1, Put or P1s(PICUS).
The latter is only used for a successful flight test with an FE/CRE
or SNY where you don't record the flight time in any of the total columns.