Fill/Drain & Overflow fittings are mostly used for ebb & flow and drip systems. In an ebb & flow system, you pump the water from your reservoir up to your flood table. When the water level gets to the level of the overflow fitting, it spill back into the reservoir from the overflow fitting. In a drip system, you would just use a Fill/Drain fitting to drain the water back to your reservoir from the flood table.
The fittings accept 1/2 or 3/4" tubing. People tend to use 1/2" tubing to go from their pump to the flood table, and 3/4" from the overflow fitting back to the reservoir such as Green Gold. We sell all of the major combinations. Individual screens and extensions are also available. Each extension adds 1 1/4'' of height.