Luckily, the Harvester is a plug in hybrid

.
Given, there are two categories of plug in hybrid (Serial, and Parallel). And the harvester is going to be a Serial plug in hybrid, which at the moment, is less common.
The real difference is that in a serial plug in hybrid, the gas motor doesn't directly drive the wheels of the vehicle (no mechanical connection between the engine and the wheels, no Transmission, no driveshafts, nothing). Instead it simply generates electricity to drive the wheels. This is simpler, and lets people design the vehicle more like an EV.
A parallel hybrid, can power the wheels with both the electric motor, and the gas engine at the same time (ie, in "parallel"). It has some benefit, because you can use a smaller electric motor/and or get more power output, and otherwise basically use a normal combustion vehicle drivetrain.
Some in the industry have started calling Series hybrids "extended range electric vehicles", but there really isn't anything specific to the architecture that makes it any more or less "extended range" than a parallel hybrid. Instead, they just tend to have larger battery packs, and they end up calling the gas engine the "range extender".
Anyway, the Harvester version of the scouts is supposed to have 150 miles of EV only range, and then fire up the gas engine and drive until the gas tank runs dry (and repeat as you'd like), for a total of 500 miles of range. Once you fill up, you'll only go another ~350 miles though (assuming you didn't also recharge the battery to 100%).
Most parallel PHEV's, have a range of ~30-40 miles or so in the USA... because there was some tax credit stuff that you didn't qualify for unless you hit a minimum of 30 miles of range. But they "could" have larger batteries if they wanted.