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 POND DC 1700: Max Flow 1700 l/h, Max Head 2.1 m
Seltz POND D DC 2700: Max Flow 2700 l/h, Max Head 2.5 m
Seltz POND D DC 3800: Max Flow 3800 l/h, Max Head 3.2 m
This means that the pump will only pump upto this height.Control of flow is easily done by adjusting the electronic flow control.