Joaquin,
Global policies are essentially fragments applied before or after every service policy in the system. Instead of putting the logic in each individual service, you can put them in a 'global policy'. We have 6 identified ones in the Gateway:
message-received
pre-security
pre-service
post-service
post-security
message-completed
More information can be found here:
Global Policy Fragments - CA API Gateway - 9.2 - CA Technologies Documentation
I hope this helps.
-Alec Daniello
APIM Support