From their announcement:
The Thorn Heavy Industries Utility Carapace was originally intended for deep immersion in hostile industrial environments. Designed to withstand physical loads of pressure and gravity that would kill unprotected bodies, the Firebrand found they offer excellent protection from the advanced firepower of their enemies. Once retro-fitted with devastating weaponry, these suits are ideal for the Firebrand's battle with the Vanguard.
There are two variants of these units - the Maul and Mattock patterns. Both have the same central chassis, but offer different weapon and arm combinations. Both are supplied with the open and closed hatches, and the separate pilot heads.