What is the difference between role=dialog and role=alertdialog?

Question

According to the WAI-ARIA specification both roles should have:

• Focus on first focusable element
• User should not be able to leave dialog
• Should have proper aria-label set
• Should be used to interrupt flow and should require to take some action e.g. to click a button or to type something

The only difference I see is that alertdialog should have aria-describedby set.

This leads me to a question. Whats the practical difference between role=dialog and role=alertdialog? When shall we use one or another?

Answer 1

As described here, alertdialog is a subtype of dialog.

What differentiate them is their role. As the name implies, alertdialogs should be used when the context requires the user's immediate attention (i.e in case of alerts, warnings or errors).

edit: corrected the explanation thanks to @cjl750 comment.