Radiation damage works as follows:

10 Rads = 1% of MaxHP

1000 Rads = 100% of MaxHP = Death

Weapons that do Radiation Damage will always have 2 damage values. The base damage of the weapon, and the "bonus* Radiation Damage. However, when you strike an enemy, only one of the two damages will be applied, whichever is higher.

Note

The Gamma Gun is listed as having 110 damage "pure" radiation damage, but this is a lie. 100 of the damage is Radiation Damage, and the other 10 is "physical damage". They are two separate sources.

Here are examples of how damage is applied in game:

Legendary Irradiated Combat Knife: 15 Physical Damage / 50 Radiation Damage

50 Radiation Damage = 5% of MaxHP in damage.

If the enemy has 100 HP, then that 50 Radiation Damage would do only 5 damage.

15 Physical > 5 Radiation, so ONLY the 15 Physical Damage will be applied because it is higher.

Now, let's say the enemy has 1000 MaxHP.

50 Radiation Damage = 50 Damage

Now, if you strike this enemy with your knife, they will receive 50 Damage, and the 15 Physical will not be applied because the Radiation Damage on the combat knife is higher.

So in conclusion, only ONE or the OTHER will be applied. Not both.

This has all been confirmed 100% through extensive in-game testing along with console commands.

Interesting

What about the double values on some weapons with energy instead of radiation?

What I would like to know if everything can take the radiation damage (if it's the highest), or just humans?

That is such a weird way to do it, that it must be a bug. Why would radiated bullets suddenly stop delivering physical damage or vice versa? Doesn't make sense at all.

That is quite simple and logical...and it only affects radiation weapons.

Following the scenario with an enemy with 100HP and your radiation knife:

Hitting him damages him for 15 and reduces his max HP by 5, so now he is at 85/95...the radiation damage has no discernable effect.

For enemies with more health this effect reverses, since the reduction in max HP is higher than the damage done (before reducing the max HP)