Training and likely injuries in the environment. A lot of kits focus on penetrating injuries, like gunshot wounds with massive bandages, tourniquets etc. Most of my experience has been with fractures, esp. legs, in the wilderness. Hence lots of splints, etc. and emphasis on less obvious trauma (massive internal bleeding, less obvious spinal injuries, and the like.

A good kit should include a stethoscope, BP cuff, good bandages, at least a splint or two (although these can be improvised). Tourniquets and the like if you are around firearms and other mechanisms that will induce penetrating injuries. A C collar will be useful.

In any case, monitor and record vital signs and transport rapidly to definitive care