I'm pretty sure this is a breaker.
I have a very similar one installed on my truck; this was a DIY job that I did myself, but I did so after extensive research and I believe this is the best way to run power from one part of the truck to the other, especially if you have dual batteries or a battery mounted in a place that isn't the engine compartment.
A bit more detail: Your alternator will be a "Smart" alternator, which means it will put out variable voltage depending on the charge state of your battery. That energy has to get into your battery -- either from the alternator into your primary battery, or from your primary battery to your secondary battery. This is high volt/high amp stuff - you want to be able to charge really efficiently, so you want to use cables that are generous enough to allow for that to maximize your recharge rate, and you also sometimes have really heavy draws (like a winch) so you also need cables that are sized generously enough for that draw out of the battery. These "big charge/big draws" are why that particular breaker is 150 amp and why they appear to be using 4 AWG cables.
So anyway, energy is coming from Alternator through a big cable. It should, as a best practice, go through a breaker as close to the source as you can reasonable make it work. The reason for this is that if, after 150,000 kms, an unnoticed hard part wears away the shielding of your really big cables and shorts out, it'll trip the breaker as soon as the short is detected. If you put the breaker at the other end -- away from the source of power and close to where the power is being saved up in the battery -- then you've got quite a few feet of cables that, should they be compromised and short, will be un-breakered and thus likely to start a major fire.
So to reiterate with word pictures:
Alternator --- Big Breaker ------------------------------------------------------Primary Battery under the seat
OR
Alternator ----- Battery under the hood----big breaker-----------------------------------------Second Battery elsewhere (common with a lot of dual battery builds from other manufacturers).
OR
Alternator--- Big Breaker ------------------------------------------------------------------Primary Battery under the Seat ---- 2nd Big Breaker --- Secondary Battery under the seat. (I believe this is the Ineos design).
Either way that black box is 99% certain to be a breaker for the application I've described here.
EDIT TO ADD MORE INFO:
The other reason I think the above is the "Ignition Controlled" language. This is also why I brought up the smart alternators earlier and then forgot to mention it again!
With "old school" alternators -- they generally put out a steady voltage for a given RPM -- you could use a simple solenoid to charge two batteries off the alternator. That solenoid could be ignition-triggered, so that when the engine is running, the alternator charges both your "primary" battery (Referred to as your "starting" battery) and your "secondary" battery (often referred to as your "house" battery). Then, when the engine is off, the solenoid is triggered the other way, which isolates your "house" battery from your "starting" battery, which is important - you don't want to fish that last beer out of your fridge only to find that your vehicle won't start because you drew down your batteries too far!
With modern smart alternators, a simple "Dumb" solenoid doesn't work as well -- the alternators, as I said, adjust their output "automagically" -- I won't get into the details here because it might as well be magic, but suffice it to say that modern alternators rely on computers to know how much energy to put out. Therefore, if you have a second battery in a modern vehicle, you will likely need to install a computer somewhere in the system to ensure both batteries are being charged when the engine is on, and that they are isolated when the engine is off. These computers are fairly cheap, mostly plug-and-play, and available all over the place - Redarc, Renogy, and CTEK are three of the main brands who sell these "DC to DC chargers". It is likely that the alternator feeds into that breaker, then there's a long run of cable to get to the rear seaet, where it connects to the primary battery and some form of DC to DC charger is installed when one options a second battery under the seat.
So in short: This is why the breaker is ignition controlled - you want your second battery to charge when the engine is running, but be isolated when it's not, and for modern trucks that requires fancy computers that are very sensitive to unregulated voltage/amps and thus control that energy very specifically.
