How many primary domains can exist in an instance?

In the context of a ServiceNow instance, there can only be one primary domain. The primary domain acts as the top-level domain for the instance and is essential in establishing the framework for domain separation. This model allows for the organizational classification of records and differentiating data access across various domains based on the business's operational needs.

Having a single primary domain is integral because it ensures that there is a clear hierarchy and control structure in place for managing domain-specific data and user permissions. This prevents any complications that could arise from multiple primary domains coexisting within the same instance, which could confuse users and complicate data management practices.

Secondary domains can exist and help in further categorizing the associated data and security contexts, but the singular nature of the primary domain is vital for maintaining a centralized and coherent operational structure within the ServiceNow platform.