This article provides information about the FaxMaker user management system and and its interaction with multiple domains and use with FaxMaker.
FaxMaker uses the service account (GFI FaxMaker Faxserver) and its permissions to be able to access the domain and the Active Directory in that domain. Using those domain permissions, FaxMaker allows the you to create licensed users from multiple domains, as long the domains belongs to the same forest and are in parent-child relationship. The service account would have to be in the parent domain to have access to both the parent and the child domain.
FaxMaker also has more options for users who own an unlimited user license. Please see the additional user options for Faxmaker unlimited user licenses article.