Since we have a couple of threads on this topic, I figured I would throw in one more.

I have a utility trailer with a 4pin flat light cable, A boat with a 4 pin round connector and an old horse trailer with a vehicle side connector 7 pin round connector and an UGLY hack job to get the connector to 4 wire flat cable. My tow vehicle has a 4 wire flat connector that I hate, hate hate. Always hanging there, hitting the ground, never stays in its holder, get replaced at least once a year. What I would like to do is move all the trailers and truck to one of the round connectors, but what one?
What are the various round connectors used for, I see 4,5,6,7 and 9 wire trailer connectors. What are each used for?