In FortiOS, the Internet Service Database (ISDB) objects are treated as a special category within firewall policies. Due to their dynamic nature and complex definitions (which can include multiple IP addresses, ports, and protocols), they cannot be combined with other object types like IP addresses, FQDNs, or user groups within the same source or destination field. When an Internet Service is selected, the policy matching engine is dedicated to that specific object type for that field, and the GUI/CLI prevents the addition of any other objects to maintain policy integrity and avoid matching conflicts.

A. IP address: An IP address object is a standard address type and cannot be added to the source field once an Internet Service object has been selected. C. User or User Group: User and User Group objects are used for identity-based policies and cannot be mixed with Internet Service objects in the source field. D. FQDN address: An FQDN address object is another type of standard address object and is not permitted in the source field alongside an Internet Service object.

1. Fortinet FortiOS 7.4.0 Administration Guide: In the section describing firewall policy components, it explicitly states the limitation. "When an Internet Service is selected as the source or destination in a firewall policy, other types of addresses cannot be added." (See FortiOS Administration Guide 7.4.0, Page 818, "Firewall policies > Internet Service").

2. Fortinet FortiOS 7.2.0 Administration Guide: This behavior is consistent across versions. The guide notes, "When an Internet Service is selected as the source or destination in a firewall policy, other types of addresses cannot be added." (See FortiOS Administration Guide 7.2.0, Page 778, "Firewall policies > Internet Service").

3. Fortinet Cookbook: In a recipe detailing the use of the Internet Service Database, the configuration steps and notes reinforce this rule. "Note that you cannot add any other addresses to the policy when using an Internet Service." (See Fortinet Cookbook, "Internet Service Database (ISDB)", Version 7.4.0, Configuration section).