Mirakl is not a marketplace — it is the infrastructure marketplaces run on
What sets Mirakl apart from other connections: it is not a sales channel itself but the infrastructure many retailers run their own marketplace on. That is why setup asks for an operator address — you state which retailer's Mirakl instance you are connecting to. Leave it empty and a default applies, but if you work with several operators it has to be written explicitly.
Because one user can have access to several shops, there is an optional shop ID field too. Fill it in and every request is confined to a single shop — this is the field that prevents stock being written to the wrong shop on multi-shop accounts.