The documentation for the Master Arm module is still being written and will be available on the Savage wiki soon.
Unfortunately the module is a bit complicated to attempt to explain fully in this thread but I will attempt to explain the basics so you can use the module in your missions.
The action is added to players at the start of the mission if the master arm module is placed in the mission.
For the action to be available to players they must meet the following conditions:
They must be in an aircraft vehicle.
They must be going below 5kmph.
They must be the pilot.
They must be within 25m of an object that has been synced to a master arm module.
So place objects at the locations where you would like players to be able to access the menu and sync those objects to the module.
Inside the module attributes enter the class names of objects which you would like to act as Ammo/Fuel/Repair vehicles.
An Ammo/Fuel/Repair vehicle must be beside the player when they are in the Master Arm module for the Rearm/Refuel/Repair actions to be available, this also means that a rearm vehicle is required to change the weapon loadout of the vehicle.
Attributes should be formatted as an array like so, ["Land_BeachBooth_01_F"]