Agency vs. Nonagency securities

pg. 246:

“Nonagency CMOs are often referred to as whole-loan CMOs, because unsecuritized loans are called whole loans. In other words, the collateral behind nonagency CMOs is a pool of loans rather than passthrough securities”

I thought passthrough loans were a pool of loans? Could someone explain to me the difference between the CMO’s pool of loans rather than the agency CMO’s passthrough securities?