SOLiD mandates the use of APC (Angle Polished Connectors) type for fiber termination to mitigate issues with back reflection that will adversely affect the signal due to the high return loss. Although APC typically has poorer insertion loss and in some cases similar to UPC (Ultra Polished Connectors), APC nominally offers atleast a 10x better reflection spec than UPC. For SOLiD, our requirement is -55 dBc or better, which is also noted in our spec sheets.
The 8 degree angle in the APC also offers tighter connections at the end face and will provide much more robust performance in complex multi signal deployments.
While UPCs have been used in deployments especially between patch panels even though SOLiD strongly advises against that, it’s at the risk of affecting system performance. All of SOLiD's hardware has only APC ports and should only be mated with fiber with APC connectors only. I suspect there are systems out there with existing or impending problems and it hasn’t been noticed yet.
Below is a quick illustration (courtesy Belden) that shows the reflection characteristic via the red arrow. With APC type, most of the reflection is scattered or absorbed via the cladding.