Super Leaderboards are generally used as regular ad units in multi-sized ad slots.
They can also be used as Pushdown ad units. This means the 970×90 appears across the top of a site when it first loads, pushing down the content. The larger 970×250 Billboard unit is generally used for this however as it is a much more successful ad unit.
Pushdown Super Leaderboard ads should be entirely closeable via a “Close X” button, leaving only a “Show Ad” button remaining that will expand again when clicked. This is up to the individual publisher, however. Alternately these ads often auto-close after 30 seconds instead.