I have never heard the acronym EUSERC used within SCE or SDG&E territory.
It is the opposite for me...ALL
Service Equipment shall be EUSERC Listed.
More precise, Approved Service Equipment shall be from a list of approved EUSERC Drawings.
This is standard with:
APU (Anaheim Public Utilities),
Burbank W&P (Water & Power),
Vendors typically indicate the PoCo + EUSERC + Drawing Numbers... i.e.: SCE - EUSERC Drawing No. 325