Using the image above, an example of delivering goods via a Ship, Optimization would find the fastest route while still maintaining the Ship as the mode of transportation. This is akin to Topology Optimization yielding a single outcome for a given setup.
Generative Design is more of a Design Exploration such that we could look at different modes of transportation such as trucks, trains and planes. All of these will vary the amount of cargo and the rate at which they can be transported. This would give you the ability to balance multiple factors at once. For Generative Design this may be Manufacturing Method, Materials, Mass, Displacement etc.
