CustomFieldReferenceValue:
    | "approval-flow"
    | "approval-rule"
    | "associate-role"
    | "business-unit"
    | "cart"
    | "cart-discount"
    | "category"
    | "channel"
    | "customer"
    | "customer-group"
    | "key-value-document"
    | "order"
    | "product"
    | "product-type"
    | "review"
    | "shipping-method"
    | "state"
    | "zone"
    | string

Defines which resource type a CustomFieldReferenceType can reference.