Before the Implementation of IFS cycle 46r1, there were two separate CAMS Global archived analysis and forecast daily representations of SO2 and SO4: one as part of the aerosol scheme, and one as part of the gas-phase chemistry.
For aerosol-related purposes, it's recommended to use "aermr11" as sulphate aerosol, and "aermr12" as its SO2 precursor.
For gas chemistry purposes, the "so2" parameter represents SO2 in a way that's fully coupled with the rest of the gas chemistry (oxidants etc.). However, the accompanying "so4" parameter is not generally recommended for use.
From the Implementation of IFS cycle 46r1 in July 2019 onwards, these representations have been unified so that the aerosol and chemistry schemes are properly coupled, and only "so2" and "aerm11" should be used.
