Global policies are used to define rules for traffic that is not associated with any particular zone. This type of policy is evaluated first, before any rules related to specific zones are evaluated.
For more detailed information about global policies, refer to the Juniper Networks Security Policy Overview guide, which can be found at https://www.juniper.net/documentation/en_US/junos/topics/reference/security-policy-overview.html. The guide provides an overview of the Juniper Networks security policy architecture, as well as detailed descriptions of the different types of policies and how they are evaluated.