RadioResourceConfigDedicated IE carries information related to establishment/modification/release of data radio bearers and as well as info related to establishment/modification of signalling radio bearers. RadioResourceConfigDedicated IE also carries mac-MainConfig, sps-Config, physicalConfigDedicated sub informations which are common for all SRBs as well DRBs. Reason being, these things are not included in srb-ToAddMod list explicitly. SRB's data does not need any header compression, discard timer at all, that's why PDCP-Config is not present in srb-ToAddMod IE but it considers as RLC AM mode that's why RLC-Configuration is present for SRBs.
I tried to explain things as per my understanding.