Logic (Module 2) allows us to describe the world as it is now. Temporal logic also lets us talk about how the world will be. The topic of Module 3, Planning, focuses on how to choose what actions to take in terms of what pre-requisite world condition must be met for an action to be feasible and what changes will the action impose on the state of the world.
The ultimate goal is to take the world from a (well-described) initial state to a desired (target) state through a sequence of actions that are all valid in the sense described in the above paragraph. Furthermore, the shortest such sequence is of special interest. One way to search for short sequences is with heuristics (Module 1), but also formal optimization approaches (like those of CCCS 640) are common for smaller problem instances.