The command handler, formalised
A handler coordinates one behavior through ports; it does not make the domain decision.
You already met the command handler in Module 2: the single entry point for one behaviour, talking to the world only through ports. Now we can state its job precisely.
A command handler receives one command, interacts with 3rd parties through a port, such as the database, asks the domain to make the decision, and persists the result. That is all.
It coordinates; it does not decide. The actual rule — can this invite happen? — lives deeper, in the thing the handler loads. Which is the next component, and the most important one in the module.