1. If Auto-matching is selected, RawTherapee will try to the following , in this sequence:
a. locate a DCP profile in /dcpprofiles or
b. if DCP is not found, locate an ICC profile in /iccprofiles or
c. if DCP and ICC are not found, revert to hard-coded camera matrix.
Matching is dome by camera name, exactly as is shown in the info sectinon in the editor. E.g. "Canon EOS 5D Mark III.dcp"
Camera matrix values are generally from dcraw, but in some cases have custom overrides based on measurements with ColorChecker24
2. Referenced WB values are for mid-day daylight. You are right, "daylight" is a very broad term... it varies with day time, season and geo location. If you need further info, I'd suggest to post this question in rawtherapee's own forum. DrSlony & Jacques might help.