Packet direction is determined by whether a packet is entering or leaving any Packet Forwarding Engine interface (with respect to the forwarding plane) on which the interface-service statement is applied. This is similar to the input and output direction for stateless firewall filters.
The match direction can also depend on the network topology. For example, you might route all the external traffic through one interface that is used to protect the other interfaces on the router, and configure various services on this interface specifically. Alternatively, you might use one interface for priority traffic and configure special services on it, but not care about protecting traffic on the other interfaces.