Good Question! The difference is actually quite straight forward. A reducing coupling is designed to accept pipes into either end. For example a 15mm to 10mm reducing coupling accepts a 15mm pipe into one end and 10mm pipe into the other.
A 15mm to 10mm fitting reducer accepts a 10mm pipe into one end and the other end goes inside a 15mm fitting such as an elbow or coupling or tee. It is always the larger end that will fit into the fitting and the smaller end will always accept the pipe.