Fixed Income Credit Ratings anybody, specific, unsecured, its

mp3bu

I have a doubt from the schewser that in LOS 42.c its saying that Issuer ratings are based on the overall creditworthiness of the company.And the issuers are rated on their senior unsecured debt.
First it talks abt the overall performance of the company then concentrated on the specific issue. Can anybody explain this???
Thanks in advance