With the development of technology, Applicable fiber connector end faces are PC, UPC, APC, etc., Physical Contact (PC), Ultra Physical Contact (UPC), and Angled Physical Contact (APC).
APC connectors feature a fiber end face that is polished at an 8-degree angle, while UPC connectors are polished with no angle. UPC connectors are not exactly flat however, they have a slight curvature for better core alignment. With UPC connectors, any reflected light is reflected straight back towards the light source. However, the angled endface of the APC connector causes reflected light to reflect at an angle into the cladding versus straight back toward the source. This causes some differences in return loss, which is a measurement of reflected light that is expressed as a negative dB value (the higher the value, the better). Industry standards recommend that UPC connector return loss should be -50dB or greater, while APC connector return loss should be -60dB or greater.
What should be noted is that APC and UPC connectors cannot be mated, in case to cause poor performance or to destroy both connectors. However, when choosing the right connector for your specified application, factors like cost and simplicity should also be considered seriously, not just optical performance. So, it actually depends on the real circumstances and various factors.