The major difference in terms of appearance is the fiber endface. PC connectors and UPC connectors are both polished with no angle, though endface of UPC has a slight curvature. Instead, an APC connector is featured with 8° angle endface.
Another obvious difference is the color. It is visible to distinguish which type of polish style the connector features by the color of the connector body.
It used to be hard to achieve low insertion loss using APC connectors owing to the air gap in the apex offsets, yet nowadays the improvement of connector design and manufacturing push APC connectors to perform better. The insertion difference between APC and UPC has been diminished.
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.
Since some applications are more sensitive to return loss, APC connectors are preferred in these fields, for instance, in higher wavelength range like those used for RF video signals, especially FTTX applications and for applications such as passive optical networks and other WDM systems using high wavelength via single mode fiber.
For those applications where return loss is not paid much attention to, UPC or PC comes into play. PC connectors are generally seen in telecom operators’ equipment, while UPC connectors are popular in digital TV, telephony and data systems.