Today a Building Block can only declare a dependency on a single instance of

another Building Block Definition via "Parent Building Block Output" - one

output value from one parent. There is no way to relate a Building Block to

an arbitrary, optional number of instances of another definition and collect

a specific output from each as a list.

Example: a GKE cluster Building Block creates a node pool with a service

account that needs read access to a variable number of Artifact Registry

repositories, each provisioned as its own Building Block. Today this

requires deploying a separate "Relation" Building Block per repository link,

which does not scale and quickly becomes confusing to manage.

Allow a Building Block input to be sourced from N related instances of

another Building Block Definition (an optional, list-based relation),

taking a chosen output (e.g. "id") from each related instance and passing

them as a list input (e.g. ). The Building Block

implementation can then iterate the list.