I believe that SkyWest employees traveling on Delta metal have a standby code of S3C and follow Delta retirees S3B on the Airport Standby List.
When Delta retirees travel on Sky West metal the codes are - Sky West being S3C and Delta being S3CR.

Airlines; not only Delta are doing their level best to confuse the boarding priority order even within their own code share partners.
Why it could not be simpler than for pleasure board with your senority date - for business board with the priority need: i.e. deadheading crews
to operate a flight - maintenance to fix equipment before board meetings and sales parties.

There are 46 standby codes (revenue and non-rev) it is no wonder it is a can of worms - and even more so for our revenue customers !

An interesting forum to read is:
Delta Air Lines | SkyMiles - FlyerTalk Forums

Even are loyal customers are equally confused and upset.

Oh what a tangled web we weave to confuse even ourselves as well as the public !