You are probably referring to a CSP, right? The centralized service port (CSP) is a special purpose port to enable centralized services for mainly VLAN-based networks. North-south service insertion is another use case that requires a centralized service port to connect partner appliance and redirect north-south traffic for partner services.
This means you can, for example, have the default gateway of a VLAN be the CSP interface of an NSX-T router.
I don't know about the CSP...but when I go to edit a T0 gateway in NSX Manager and click "Add Interface", I have the option of creating two types of interfaces: service or external. The service interface is the one I'm wondering about.
The service interface is the new name for CSP.