Before you create your companies, you can agree as a group to include hazardous terrain in your upcoming battlefield. When you build your frames, you can give them an appropriate environmental system.
It counts against your limit of 4 systems per mobile frame. In order to function effectively in hostile terrain, a mobile frame needs the appropriate environmental system. Without it, when you roll dice reduce your white dies by one.
Examples: When fighting in the vacuum of space on a ship's hull, a frame might need magnetized soles. When wading through a swamp with irritating gases, a sealed air system is important. Whilst in subzero conditions, a thermal unit would be advisable.