Some probability of default are issuer-weighted and some are volume-weighted. I don't understand what this means. I had a look into Moody's documentation available here:
However I still don't understand the part explaining it:
With the model described in this paper, users are able to forecast the probability of rating transitions and default at the issuer level and at the portfolio level. The portfolio forecast can be equally-weighted across issuers as well as volume weighted.