From the certbot documentation for --preferred-chain:
prefer the chain whose topmost certificate was issued from this Subject Common Name.
From Providing a longer certificate chain by default also linked above:
This chain will consist of three certificates, instead of the current two:
- End-entity certificate (aka leaf certificate), signed by R3
- R3, signed by ISRG Root X1
- ISRG Root X1, signed by DST Root CA X3
Our API will also offer an "alternate" chain, which you may configure your ACME client to select instead:
- End-entity certificate, signed by R3
- R3, signed by ISRG Root X1
The possible values for the --preferred-chain clearly indicated by the "signed by" followed by the required common name.
Although perhaps some explanation about what a Common Name actually is might be helpful, it's not rocket science.