You are a Wi-Fi provider and host multiple domains.

Realms are a way to delegate user accounts, user groups and permissions per domain when they are authenticating on a single FortiAuthenticator device. A realm is a logical grouping of users and groups based on a common attribute, such as a domain name or an IP address range. Realms allow administrators to apply different authentication policies and settings to different groups of users based on their realm membership.

References: https://docs.fortinet.com/document/fortiauthenticator/6.4.0/administration-guide/906179/user-management#realms