Are you aware of the very good tips and notes sections for the Swahili tree?
I can access it via the "bulb" sign per skill in the website tree, but not from the Android App tree (status 20-Jan-2019).
I hope that the tips and notes section fo skill "Locatives" can answer you question:
iko is when something has a general or nonspecific location and -ko is commonly used when asking a question about the location of something. -po means you have a definite location. As something becomes more specific, it tends to be a -po. I'm in Texas would be more -ko, but being in Dallas is more specific - thus it would be -po.