It's very common for different processes to communicate with each other. At the most basic level, that communication might take the form of something as simple as one function or method calling another from somewhere in the code they share. As processes scale outward, though, especially if they are distributed across separate physical or virtual devices, those communication chains will often get more complex themselves, sometimes even requiring dedicated communications protocols. Similar communication-process complexities can also surface, even in relatively uncomplicated systems, if there are interprocess dependencies that need to be accounted for.
In pretty much any scenario where the communication mechanism between two processes is more complicated than something at the level of methods calling other methods, or perhaps a method or process writing...