Here is what UESPWiki has to say about the http://www.uesp.net/wiki/Skyrim:Essential_NPCs:
Spoiler There are several NPCs in Skyrim who at some point during gameplay are essential. In effect, this means they cannot be killed under any condition (unlike http://www.uesp.net/wiki/Skyrim:Protected_NPCs, who can be killed normally by the player, but not by others), and when their health bar fully depletes, they will merely fall into a state of recovery for a short time before standing back up with full health. Unlike http://www.uesp.net/wiki/Oblivion:Oblivion, it is not possible to tell whether an NPC is essential without attempting to kill them or by using the level 100 http://www.uesp.net/wiki/Skyrim:Pickpocket perk http://www.uesp.net/wiki/Skyrim:Perfect_Touch, which allows the player to remove equipped items; in the case of essential NPCs, however, the player will not be able to use this perk on them. NPCs are usually only set to essential if they are of high importance to a later quest (some of these essential NPCs have http://www.uesp.net/wiki/Skyrim:Scripted_Deaths during, or at the end of, the quest).
Some NPCs in Skyrim are always essential and can never be killed, even after their associated quests are completed. Notable examples include the Imperial legates at the Imperial camps and named Stormcloak officers at the Stormcloak camps.
http://www.uesp.net/wiki/Skyrim:Child are not listed here because they use a tag other than essential to prevent them from being killed. This tag is a part of their race and, with the exception of http://www.uesp.net/wiki/Skyrim:Babette and http://www.uesp.net/wiki/Skyrim:Sofiehttp://www.uesp.net/wiki/Skyrim:Hearthfire, these two tags are never both found on the same NPC.
And http://www.uesp.net/wiki/Skyrim:Protected_NPCs:
Spoiler Several NPCs in Skyrim are protected at some point during gameplay. This means that when their health depletes (usually down to 10% of its maximum, but this will occur even if the NPC's health would have been reduced to zero), they enter a state called "bleedout", where they fall to their knees and are ignored by all hostile entities until their health regenerates. Unlike the similar http://www.uesp.net/wiki/Skyrim:Essential_NPCs status, they can still be killed if they are attacked by the player, where they will die as easily as non-protected NPCs.
A permanent http://www.uesp.net/wiki/Skyrim:Followers accompanying the player is set as protected, regardless of whether or not they are normally. This also applies to the player's http://www.uesp.net/wiki/Skyrim:Marriage and anyone http://www.uesp.net/wiki/Skyrim:Rebuilding_the_Blades into the http://www.uesp.net/wiki/Skyrim:Blades.
To be killed, as of the 1.9 Patch, a protected NPC must have his or her health fall to zero from a direct hit by the player. An important keyword in this definition is "direct." This can sometimes lead to your follower or spouse surviving attacks from you when you don't expect them to. Namely ...
Fall damage from the http://www.uesp.net/wiki/Skyrim:Unrelenting_Force shout will not kill a protected NPC. A poison slipped into your spouse's or inactive follower's pocket (Note: Only inactive followers can be pickpocketed) using the http://www.uesp.net/wiki/Skyrim:Poisoned will only bring the Protected NPC into bleedout, even if that same poison would kill the protected NPC if it were applied to the Player's weapon.
For a list of protected NPCs see http://www.uesp.net/wiki/Category:Skyrim-Protected_NPCs and for a list of protected creatures see http://www.uesp.net/wiki/Category:Skyrim-Protected_Creatures.
The crucial part for the Protected tag description being:
Based on this, it wouldn't matter if a Deathclaw attack would normally rip them to -500% health in one blow, it still wouldn't have that effect, because they will not go below about 10% from NPC attacks (or at least from a single NPC attack) and once they are at ~10% they can only go lower as a result of damage caused by the player (with splash damage or possibly things like land mines, turrets, etc., possibly counting as "damage caused by the player").
An important caveat is that, it got changed a bit with the final patch for Skyrim:
So, in the final vanilla version of Skryim, the "splash" or "overflow" damage from player being able to kill "Protected NPCs" was removed. Protected NPCs would never go below some low threshold (around 10% or maybe less) from ANY source of damage other than a direct hit by the player.
I am certainly no expert on this stuff and do not pretend that the way it works in Skyrim is necessarily proof of how it is working in Fallout 4. But my limited observations up to this point suggest to me that the same two (well three if you count the additional tag on children) tags are being used in FO4 and that they work just about the same as they did in Skyrim:
Essential: cannot die by any cause.
Protected: cannot die by NPC caused damage. If NPC caused damage would take them to zero or less than zero health, they go to a low level (~10% or thereabouts), go into a sitting animation and remain in this "bleed out" status until hostiles are clear from their area, at which point they start to regenerate and then eventually get back up and behave normally. Direct damage (and maybe 'indirect' or 'proxy' damage) from the player can nonetheless kill a protected NPC at any time.
Hopefully I am not being misleading, and I don't intend what I'm saying to be the "final word," just trying to clarify what is probably/maybe going on in FO4 based on how it worked in their most recent game with the sister version of the engine, Skyrim.
It may be possible as of this time (and even without the GECK) to do enough detective work with FO4edit, and/or just in-game tests to determine if it works differently in FO4 than it did in the final version of Skyrim.