What flow is available from the pump and what is meant by "Max Head"?

The Maximum Flow of the product is based up the flow of water as it leaves the pump.
Maximum Head is where the pump is used with tubing to pump to a higher tank or piece of equipment, therefore:
Seltz D AC 6000: Max Flow 6000 l/h, Max Head 3.8 m
Seltz D AC 9000: Max Flow 9000 l/h, Max Head 4.4 m
Seltz D AC 12000: Max Flow 12000 l/h, Max Head 4.8 m
This means that the pump will only pump up to this height. Control of flow is easily done by adjusting the electronic flow control.