Here is a reference from Juniper on it with more details if you like :
"For example, if a policy lookup is performed, regular security policies have priority over global policies. Therefore, in a policy lookup, regular security policies are searched first and if there is no match, global policy lookup is performed."
https://www.juniper.net/techpubs/en_US/junos13.1/topics/concept/security-policy-global-policy-overview.html