APC refers to Angled Physical Connect, the ferrule endface radius of which is polished at an 8°angel, minimizing the back reflection as a result. Since adding the angled endface, the reflected light will leak out into the cladding instead of staying in the fiber core.
APC connectors should only be mated with other angled-polished connectors rather than non-angled polished connectors, or it will cause very high insertion loss
APC/UPC fiber connector difference
Performance Difference
As for the performance of return loss, you may have noticed APC connectors have the lowest back reflection of all the fiber connectors mentioned owing to the angled endface. For this reason, the values of the return loss are different. According to the industry standards, the return loss of PC, UPC, and APC connectors is respectively about -40dB, -50dB, and -60dB or higher. The higher the return loss is, the lower the reflection and the better the connector performance will be. It proves that APC connectors are higher-performing among the connectors listed.