Looks like you've got a 3.5 ton heat pump with electric resistance heat. I'd think 50 is a bit high for the balance point. I run mine (1994 Carrier) down into the 30's in NW OH. Some of the older systems had outdoor temperature sensors, set at the balance point, that would automatically switch the system to stage 2 emergency heat once the outdoor temp hit the balance point.
You hit Eheat with electric backup, you want to make sure your house is firmly attached to the earth because you're electric meter will be spinning like a helo prop.
In case you don't know,
Balance point: The point at which the extractable heat energy gets too low for a heat pump to adequately heat your home is called the balance point.
I used to install heat pumps with electric backup but I'd stage in the electric resistance heat to supplement the heat pump as temps got closer to balance point. What I mean is the heat pump and one bank of electric resistance heat would kick in. This kept the customer from bringing in 10Kw of electric resistance heat all at once like you'll probably be doing. I doubt the electric is staged.
And if this is your first heat pump, don't freak out if you look outside and see frost on the coil.... that's normal. As long as the refrigerant lines aren't frosting (that's bad), the coil will frost during operation. The system will have a defrost cycle that triggers by run time, temperature of the coil, or a combination of both.