The smaller (inlet) fitting is used to pump nutrient into the tray (flooding)
While the larger (outlet) fitting with the extension riser and screen sits higher in the tray. Once the nutrient level in the tray reaches the height of the outlet screen, the excess nutrient can drain back to a reservoir.
The extension can be adjusted for more height, and extra extensions can be added for a deeper tray.
Fittings only, pictures are for illustration purposes.