Modal is an option to functions such as CreateDialog that specifies whether the dialog that is created should be modal to the Mathematica front end.
With the default setting Modal->False, other windows can be used even when the dialog is open.
With Modal->True, no other Mathematica windows can be used until the dialog has been closed. If there is no mechanism for closing the dialog, Mathematica will therefore effectively freeze.
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Create a modal dialog that blocks operations in other open notebooks:
Create a modal dialog that blocks operations in other open notebooks: