Is there a difference between 'on demand' and 'automatic demand' 12 volt pumps?

If so, what is the difference?

They are the same, just different ways to say it. They have an internal pressure switch that turns the motor on when pressure falls below the set pressure and off when the pressure set point is satisfied. The other type of 12VDC pumps are called bypass. They don’t have a pressure switch, they internally bypass in the head when the set pressure is satisfied.

Okay make sense. So when if ever would you use bypass?

When you don’t want to mess with finicky pressure switches that are not very reliable

In agriculture use it is often used to keep the mixture agitated.