I have only ever used SRF and had the same question. They seem to have slightly different boiling points.
https://www.amazon.com/Castrol-SRF-R.../dp/B000JL760C
https://msdspds.castrol.com/bpglis/FusionPDS.nsf/Files/5FF05F2B69DA6C4B802583D8001DB1DB/$File/React_%20SRF_%20Racing_3371486_2013-11.pdf
I was unable to find the data sheet for SRF on the Castrol website. This makes me wonder if SRF React is replacing SRF and companies are just working through the stock of SRF.