This case-sensitive code system http://hl7.org/fhir/us/directory-query/CodeSystem/OrgAliasTypeCS defines the following codes:

Code	Display	Definition
legalalternative	Legal Alternative	A pseudonym used by an organization to perform their business under a name that differs from the registered, legal name of the business, also known as a trade name, business name, or 'doing business as' (d.b.a.).
historical	Historical	Any previously used legal or trade names used by an organization since the creation of the business. Could include names used by independent organizations before a merger or other reorganization.