I’d assume it’s using the closest, or VRS from several of the others.
The uBlox and Quectel want the RTCM MSM so either the RTCM 3.1 or 3.4 forms which mostly would be the supported constellations, as each has it’s own MSM message
Presumably the AG and NAD83(1996,2011) with slightly pulled 1005 type antenna position variations for them.
Yeah, the CMR variants will be unworkable with the SparkFun boards, except probably the Septentrio based ones.
https://customersupport.septentrio.com/s/article/CMR-messages