My customer is attempting to implement the use of subnet roles when deploying their private cluster into an existing VPC, however, this feature could be titled and structured better in the documentation.
The original upstream enhancement (https://github.com/openshift/enhancements/blob/master/enhancements/installer/aws-lb-subnet-selection.md#defining-subnets-roles) explains the original use-case for this feature around being able to allow LB subnet selection. However, the use of subnet roles is beyond the LB subnet selection use-case, that is, the installer will use these roles when configuring the CAPA objects.  So this sounds more like an installer feature more so than just an Ingress and Load Balancing selection feature.

4.19, 4.20

Documentation for this feature is found here only which isn't adequate.
https://docs.redhat.com/en/documentation/openshift_container_platform/4.20/html/ingress_and_load_balancing/configuring-ingress-cluster-traffic#allocating-load-balancers

Documentation should include a short paragraph in explaining the use-case around limiting LB subnet selection using subnet roles for Ingress LB (https://docs.redhat.com/en/documentation/openshift_container_platform/4.20/html-single/ingress_and_load_balancing/index#allocating-load-balancers).
But, section 2.11.1.1 (https://docs.redhat.com/en/documentation/openshift_container_platform/4.20/html-single/ingress_and_load_balancing/index#nw-allocating-load-balancers-to-subnets-procedure_allocating-load-balancers) should be moved to "Installing a cluster on AWS in an existing VPC" 3.5 section (https://docs.redhat.com/en/documentation/openshift_container_platform/4.20/html-single/installing_on_aws/index#installing-aws-vpc), specifically under 3.5.3 (https://docs.redhat.com/en/documentation/openshift_container_platform/4.20/html-single/installing_on_aws/index#installation-initializing_installing-aws-vpc). In fact, a new 3.5.3.7 sub-section should be created where the subnet roles should be elaborated on including any install-config.yaml code snippets from 2.11.1.1 as well.
In short, any mention of Ingress LB subnet selection could be kept in section 2.11 but anything else apart from the LB subnet selection use-case should be moved to the "Installing a cluster on AWS in an existing VPC" 3.5.3 section since it does directly relate to install-config.yaml customizations.