Why patch cord, pigtail use UPC connector to replace PC connector?

When a connector is installed on the fiber end, loss will be incurred. Some light loss would be reflected back directly down the fiber towards the light source that generated it. These back reflections, or Optical Return Loss (ORL) will damage the laser light sources and also disrupt the transmitted signal.
1. Fiber connectors with different polishing types have different Optical Return Loss (see the picture below).

Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high. A high return loss is desirable and results in a lower insertion loss.

Thus the performance of UPC is perior to PC.

1. Building on the convex end-face attributes of the PC, but utilizing an extended polishing method creates an even finer fiber surface finish:UPC.

The insert core of UPC Connector can be fully butted to PC Connector. (see the picture below).  It has a higher Optical Return Loss (ORL) than a standard PC connector and allows more reliable signals in digital TV, telephony and data systems.