A full screen modal that temporary blocks interaction with the main view of a site and set the actual task in focus.
Use a takeover modal when a task needs the full attention from a user, for example when making a payment. Heavier tasks such as a multi step wizard also benefit from the focused experience of removing not needed UI chrome of the original site.
It should cover the whole viewport whilst its own content is centered, both vertically and horizontally.
If triggered from a link/button, the modal should originate from that point until filling the whole viewport to communicate hierarchy.