Domain separation is a way to separate data, processes, and administrative tasks into logically defined domains. It is best suited for Managed Service Providers (MSPs) or for companies that have a global presence with unique and varying business requirements for different parts of the world.
Before implementing domain separation, you need to carefully consider whether or not it is well suited for your organization's business requirements. Once domain separation is enabled, there is no simplified way to disable it. While it can be disabled by turning off some properties and applying some additional changes, the process of disabling it is not so clean. So surely determine whether or not it is the right approach for the organization before enabling it.
If business requirement is just to separate data for some companies or groups consider using these alternatives that can segregate data based on role, groups or other conditions:
-- Use before query business rules to prevent users from accessing certain records. Check more details in about before query BR in doc: Default before-query business rule
-- ACLs to limit data visibility among users and groups. Check here for more details about Access control list rules
-- Filters, views, view rules can be used to limit fields visible on form and list based on users and groups.
-- UI Policies can be used to hide and show fields on the form too based on conditions or script.
-- Check out this great article about: Demystifying Access Controls | ACL Basics
-- Look in docs for the more detailed explanation on the system rules mentioned above: https://docs.servicenow.com