how to calculate EIRP with antenna gain in dBiC

Hello, I have found dBiC is used for some circular polarized antenna to specify the gain rather than dBi which is more popular in antenna.Some article said that dBiC is the antenna gain compared to circular polarized isotropic antenna, but,what's the relationship between dBiC and dBi or dBd,and how to convert to dBi or dBd. Some people said that -3dB will be considered in when calculate EIRP with dBiC, also some people said that there is no specified conversion.