What is a "programmable fast path complex" in terms of a Service Routers line card?

Nokia (Alcatel) has line cards called Input/Output Module. The documentation on it says "IOMs contain the required traffic-processing fast-path complexes based on the FPx (their NPU) network processor. Fast-path complexes are responsible for buffering and QoS"

But I can only find information on fast-path in terms of DLSAM error correction.

What's a complex? What's a fast-path? (Obviously it's a fast path, but deeper than that).