Circular drain outlets are manufactured with a smooth flange for hot air welding, adhesion or mechanical fixing.
The spigot is 240mm long and is inserted down inside of the drainpipe. This gives confidence that no water can back up behind the spigot and onto the deck. The spigot is slightly cone shaped and ribbed. This helps with installation into the hole, and prevents water passing back up behind the spigot.
Always check the size required on the chart.