The regulation were changed on July 13, 2020 per 3.304(f)(3) to allow for a relax standard in which a VA can concede and verify a stressor related to service in areas of enemy hostility. The new relaxed standard was coined “fear of hostile military or terrorist activity”.

Here is the CFR reference 38 CFR 3.304(f)(3):

If a stressor claimed by a veteran is related to the veteran’s fear of hostile military or terrorist activity and a VA psychiatrist or psychologist, or a psychiatrist or psychologist with whom VA has contracted, confirms that the claimed stressor is adequate to support a diagnosis of posttraumatic stress disorder and that the veteran’s symptoms are related to the claimed stressor, in the absence of clear and convincing evidence to the contrary, and provided the claimed stressor is consistent with the places, types, and circumstances of the veteran’s service, the veteran’s lay testimony alone may establish the occurrence of the claimed in-service stressor. For purposes of this paragraph, “fear of hostile military or terrorist activity” means that a veteran experienced, witnessed, or was confronted with an event or circumstance that involved actual or threatened death or serious injury, or a threat to the physical integrity of the veteran or others, such as from an actual or potential improvised explosive device; vehicle-imbedded explosive device; incoming artillery, rocket, or mortar fire; grenade; small arms fire, including suspected sniper fire; or attack upon friendly military aircraft, and the veteran’s response to the event or circumstance involved a psychological or psycho-physiological state of fear, helplessness, or horror.

And here is how the M21 tells VA what it means, per VIII.iv.1.A.3.f.:

Fear of hostile military or terrorist activity means

• the Veteran experienced, witnessed, or was confronted with an event or circumstance that involved
  • actual or threatened death or serious injury, or
  • a threat to the physical integrity of the Veteran or others, and
• the Veteran’s response to the event or circumstances involved a psychological or psycho-physiological state of fear, helplessness, or horror.

Examples of exposure to hostile military or terrorist activity include presence at events involving

• actual or potential improvised explosive devices (IEDs)
• vehicle-embedded explosive devices
• incoming artillery, rocket, or mortar fire
• small arms fire, including suspected sniper fire, or
• attacks upon friendly aircraft.

And here are a couple of examples per VIII.iv.1.A.3.k

And VIII.iv.1.A.3.i