The PI Device Circuit Components form displays the series diode being used to protect the Power Integrations Device.
Because there is no fixed high voltage DC input, it is possible that the voltage between the drain and source of the primary Switch will drop below zero, especially when the input voltage is low (where there is a bigger chance that the supply is working in discontinuous conduction mode).
The relaxation ring that occurs on the drain to source voltage when there is no more energy stored in the transformer will have a maximum peak to peak value that is approximately 2 * VOR. Because Power Integrations uses a monolithic IC that combines both controller and Switch in a single silicon “piece” it is necessary to prevent reverse current from flowing in the parasitic anti parallel diode, and that is done by adding the diode DRING. A 200V ultrafast (trr of at least 75ns) is enough to protect the IC.
You can change the automatically selected discontinuous ring protection diode for a created design by clicking the calculator icon 
in the components box. Click change to open the Diode dialog box and edit the parameters.
The 
icon indicates that a custom parameter value has been entered. Click the icon to restore the default automatically selected value.