A domain 'contains' relationship is a good practice and a specific mechanism used to grant users in one domain visibility into data in another domain that is not in a direct parent-child hierarchical path. In many Service Provider (SP) architectures, the SP operates from its own domain, which is a peer to its customer domains (e.g., both are children of the TOP domain). In this common scenario, establishing a 'contains' relationship from the SP's domain to each customer's domain is the correct and necessary action to allow SP users to view all customer data while maintaining proper data segregation.

B. Put customer data in Global: This is incorrect as it violates the core principle of domain separation, which is to segregate customer data. Global data is visible to all users in all domains.

C. No action required: This is only true if a specific parent-child domain hierarchy is already established where the SP is in the parent domain. It is not a universally applicable practice "to allow" visibility, as action is required if this structure is not in place.

D. Setup a visibility group: While visibility groups also grant cross-domain access, they are typically used for specific sets of users or roles. A 'contains' relationship is a broader, domain-to-domain rule, making it more suitable for granting visibility to the entire SP organization.

1. ServiceNow Product Documentation - Domain contains relationships: "A contains relationship grants users in one domain visibility into another domain that is in a separate branch of the domain hierarchy. For example, you can create a contains relationship that grants users in the Service Provider domain visibility into the Customer A and Customer B domains." This directly validates the use of 'contains' relationships as a primary method for SP visibility. (ServiceNow. (2023). Domain contains relationships. ServiceNow Product Documentation for Utah release).

2. ServiceNow Product Documentation - Domain Visibility: "There are two types of domain visibility: hierarchical and granted. By default, users can see data in their own domain and in child domains. This is hierarchical visibility. You can also grant users visibility to domains outside their hierarchy." This document distinguishes between the default hierarchical visibility and explicitly "granted" visibility, for which 'contains' relationships are a key tool. (ServiceNow. (2023). Domain visibility. ServiceNow Product Documentation for Utah release).