By default, a user's domain is determined by the domain associated with their Company record. To override this inheritance, you must explicitly populate the Domain field on the user's record (sysuser table). This action directly assigns the user to the specified domain, taking precedence over the company's domain. The "Managed domain" property, which is set on the domain record itself (not the user record), is a related configuration that designates the domain as one that can be manually assigned to records and users.

B. Use a Source Script on your LDAP transform.

This is a specific implementation method for an LDAP integration, not the general platform mechanism for assigning a user's domain.

C. Set Default to true on their domain record.

The 'Default' flag on a domain record is used for users without a company or whose company has no domain; it does not override an existing company-based domain assignment.

D. Change their Company reference to a company with the desired domain.

This describes the standard, default behavior. The question explicitly asks for a method to assign a domain other than the one based on the user's company.

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1. ServiceNow Product Documentation - Domain Separation - Assign users to a domain:

The documentation states, "By default, a user's session domain is determined by the domain of their Company record. You can override this default behavior by explicitly assigning a user to a specific domain... To override the domain that is derived from the user's company, you can explicitly assign a user to a domain. To do this, you populate the Domain field on the user's record." This directly supports the core action described in the correct answer.

2. ServiceNow Product Documentation - Domain Separation - Domain Configuration:

This document explains the fields on the Domain [domain] form, including the "Managed domain" checkbox. It notes that this setting is used to indicate that the domain can be manually selected, which aligns with the process of explicitly assigning a domain to a user or record.