Dela has been built with modularization and testability in mind. As such, the system is made of multiple interoperable modules based on a set of well-defined abstractions.
The stacks display the role of each module and its relation to others. Each module can rely on zero or more of the modules below it. From the client's perspective the stack is simpler since it doesn't need to know about the complex ordering mechanism.
The execution flow displays how the system uses the modules in order to fulfill its goals. Since a client and a node have different goals, we display the perspective from those two separately.
CoSiPBFT is the actual implementation of the ordering module. This is the core part that orchestrates the addition of new blocks based on Collective Signatures. This diagrams helps to understand how the leader and its followers work together to verify and include new blocks.
Here is a simplified diagram of package dependencies, for reference:
