I needed to add this annotation otherwise I was getting errors:

Referenced "Issuer" not found: issuer.cert-manager.io "letsencrypt-prod" not found

If you create a certificate, in the issuerRef, if you do not specify a kind: then cert-manager assumes that the the issuer is of type kind: Issuer.

Since this document specifies the creation of a cluster-wide issuer of type kind: ClusterIssuer, you need to specify it here.

The cert-manager documentation says the same thing. If you look at: https://cert-manager.io/docs/tutorials/acme/dns-validation/

it says:

If you want to reference a ClusterIssuer, which is a cluster-scoped version of an Issuer, you must add kind: ClusterIssuer to the issuerRef stanza.