It seem that the documentation for parameter is milseading.
The way parameter are defined and passed between a parent component and its child use to be really well explained (I think it was in the "Parameter" page, but I'm not sure). It seems that the introduction text which use to deal with the concept is no more in the Parameter documentation, what leads to really big miscomprehension from user.
It would be really great to add again something about that at the beginning of the parameter page (before the loop example).
That is the text in the T5 refcard on that topic, which seems quite good:
A page binds the parameters of its child components. For example, a TextField properties
component's value parameter is bound to a property, or property expression, indicating what object property it will
read (when rendering) or update (when the containing form is submitted).
And there used to be something about parameter being a conduit, both read and write, between a component and it's container that was quite nice too.