[gindely]

Yeah I think nowadays a lot of people think "modal" means "a dom element positioned with respect to the viewport that prevents interaction with the rest of the webpage".

The classical sense of "a collection of related controls that prevents interaction with the rest of the application (however the collection, controls and application are implemented)" is surprisingly rare now, particularly since the older sense were often called "dialogs" (and indeed, "modal" was short for "modal dialog") - intended to emphasise that this was communication between the user of the application and the developer of the application to decide how the application should behave.

Therefore, a modal dialog should be used when the communcation couldn't have happened before and cannot happen later, and the developer of the application cannot do something intelligent and allow the user to correct it later on. (For instance, just delete the thing and let them undo it - no interaction between the developer and the user actually needs to transpire.)