Positive head means the water level in the header tank is above the shower. This means you can turn on the shower and water will flow - this flow can trigger the pump to start.
Negative head means the shower is above the water level in the header tank, or is not much below it. This means you cannot get a gravity flow through the shower to start the pump, and so the pump will need a means of starting it (eg by a pushbutton next to the shower). Once running, the flow will keep the pump running via it's flow switches - and it will turn off when you turn off the shower and stop the flow.
The pump still needs to be below the water level in the header tank, but it can then pump uphill to run the shower.
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