The round uplinks are for ST connectors, and the square uplinks are for SC connectors. SC connectors are, obviously, square and use a push-on pull-off connection, and you'll know that it is seated properly when it "clicks", or locks in place. SC connectors are usually associated more with single-mode fiber. ST connectors are more commonly associated with both single as well as multi-mode fiber. They are round in shape (looks like a BNC connector) and use a bayonet-type plunger that is seated properly by pushing and twisting a quarter turn. They can be either a keyed or spring-loaded type.
In actuality however, they can be used interchangably, without noticable signal loss or degradation.
