There are four trophies regarding killing one of every type of Machine in each category. However, there is nowhere in-game that tells you what category each Machine falls under. The most information I've been able to find was in a teaser that introduced a few of the Machines and categorized them.
The teaser categorized the Stormbird and Thunderjaw as Combat-class Machines, the Behemoth as a Transport-class Machine, and the Snapmaw as an Acquisition-class Machine.
I also assume that Watchers and Red-Eye Watchers are Recon-class Machines, for obvious reasons. However, this only covers 6 of the 24 types of Machines, excluding Tallnecks (you can't kill these) and "Corrupted Machines".
The reason I ask is that I managed to go the entire game without killing a single Broadhead, which kept me from getting the "All Acquisition machines killed" trophy, which surprised me, because I didn't realize Broadhead was considered an Acquisition-class Machine (I really don't understand what makes it that, either, but I digress...). That said, which Machines fall under the Acquisition, Combat, Recon, and Transport categories?