If memory serves me correctly:

SITI - Sold inside, ticketed inside. A ticket sold and issued in the same country
SITO - Sold inside, ticketed outside. A ticket sold in one country and issued in a country not included in the itinerary.
SOTI - Sold outside, ticketed inside. A ticket sold in one country but issued in another country on the itinerary.
SOTO - Sold outside, ticketed outside. A ticket sold and issued in a country not included in the itinerary.