That decision was to default the car into electric-only mode if its battery has any charge remaining (above that needed to operate as a conventional Prius hybrid, which it does after the battery is depleted).
It’s a simple software command, but it turns the Prius Prime from a hybrid into an electric car over its first 20 or 25 miles. That means it’s not the engine-and-electric-combined vehicle that many expected it to be.
And it sets the car apart from every other plug-in hybrid except the Chevrolet Volt, which also operates entirely in electric mode until its battery capacity is depleted.
via Green Car Reports 2017 Toyota Prius Prime: first drive of new plug-in hybrid
This software change modifies this plug in Prius into a 100 percent electric car with a backup fossil fuel engine.