They measure only default risk = potential of the borrower to default (Investmante grade or Junk). Severity of loss depends on the recovery rate of the money invested : for example a borrower can issue bonds with different priority of claims(first lien , secured debt, senior unsecured etc.) if he defaults the recovery rate will be bigger for highest priority bonds => low severity of loss. In a extreme example junior subordinated debt could have 0 recovery rate .