All the network egress traffic is shaped at the bundle or interface rate. An interface may not necessarily correspond directly to a port, and an interface could be a sub-channel of a port. As an example, Fast Ethernet could be the choice of network egress, but the leased bandwidth could still be a fraction of the port speed. In this case, it is possible to shape at the interface rate of 15 Mb/s, for example.
The same also applies to MLPPP bundles. The shaping takes place per MLPPP bundle, and the traffic is shaped at the aggregate rate of the MLPPP bundle.