Okay . . . so, does that also apply when a prone mech is propping up and firing weapons in one arm?
You can only fire weapons from an arm, while prone, if you have two arms. You only get the +2. This is the TW and BMM rule without the optional rule.
Since it sounds like TW is 2 arm prop vs TacOps 1 arm prop optional rule
That's exactly what it is. Some TO rules made it to BMM. The additional option in BMM is the Tacops 1 arm prop.
In TW, you need 2 arms. You can fire the torso and one arm. +2.
TacOps presents an optional rule, that BMM lists as an option, you can use when you have only one arm. You can only fire torso, and there's both the +2 and the +1.
The +3 is only when you have a single arm and you can fire any arm weapons then.