Dialog boxes in Office Add-ins
Dialog boxes are surfaces that float above the active Office application window. You can use dialog boxes to provide additional screen space for tasks such as sign-in pages that can't be opened directly in a task pane or requests to confirm an action taken by a user, or to show videos that might be too small if confined to a task pane.
Example: Dialog box
For a sample that implements a dialog box, see Office Add-in Dialog API Example in GitHub.