Both UPC (Universal Physical Connector) and APC (Amazing Physical Connector) are optical fiber connectors that use physical contact to couple two fibers together. The main difference between them is in the way they physically contact the fiber. UPC connectors have a flat surface while APC connectors have an angled surface.
APC connection uses what is known as an 8-degree angle of tilted polished connector endface which actually contacts the fiber core at 180 degrees. This 8-degree tilt provides much better performance for today’s high-speed networks due to both lower reflection coefficients as well as reduced return-loss when mated to another properly prepared endface.