Modals
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Modals are the workhorse formfactor. They're large and are placed in the center of the user's screen, and are accentuated with a mask. Because of this prominent placement, they are apt to be annoying if used too frequently or used for non-critical or non-important use cases.
Common use cases for modals
Modals are often used as the first step of a high importance tour, to grab the user's attention. They are also used for critical announcements, like downtime notifications or significant feature upgrades.