Your special seat request ('SSR') for window seats ('W') on all SQ flights have been confirmed ('HK') while a similar request is still pending for the MI flights ('PN'). The '1' after the HK or PN denotes a single seat.

Not sure about the rest of the codes. I suppose 'GB1G' means your itinerary was booked in the UK and through Gallileo CRS.
And the alphabets after your flight numbers are supposed to denote the fare buckets you're booked on.