What is a cross-domain user in Vault?
A Cross-Domain User is a user who has access to Vaults on a different domains without creating a new user account.
A user profile can be configured to have access to multiple Vaults under one profile.
From within a specific Vault, users can navigate to My Vaults using the Vault selector that appears in the top header area. If a user has access to Vaults across domains, Vault displays all Vaults across all domains as well as the name of the domain they belong to.
Vault Help: Cross-Domain Users and Authentication