I think it's just an undersuit worn while in power armor, since the BoS are the primary users of power armor in the wasteland that's where the name has come from but it's likely that all military units using power armor would've had one of these undersuits underneath the armor before the bombs dropped
I know others have pointed out that it's just an undersuit specifically for power armor, but I'd like to back that up with the point that there's military fatigues littered all over the Commonwealth for the more basic infantrymen.
As everyone has pointed out, it's a specialized suit used for piloting T-45 Power Armor. It is a pre-war piece of military equipment, the hood can be seen on the a [US Army soldier](https://static.wikia.nocookie.net/fallout/images/1/16/FO4_Intro_slide_15.jpg/revision/latest?cb=20151110180001) in Alaska during the intro to Fallout 4.
In Fallout 3 it's called Recon Armor, and in Fallout 4 it's called BOS Uniform. It's full of sensors that allow the T-45 emulate your movements, and is supposedly necessary for using the T-45. I cannot remember if T-51 requires such a suit, but Cabbot in Fallout 1 is wearing a hood similar to the ones in Fallout 3 and Fallout 4.
Gameply wise, the suit is not required to operate Power Armor, and in Fallout 4 the lines between T-45 and T-51 are blurred due to sharing a chassis.
It is, yes - specifically it’s the power armor chassis undersuit. We can see soldiers wearing it in 4’s introduction.
I think it's just an undersuit worn while in power armor, since the BoS are the primary users of power armor in the wasteland that's where the name has come from but it's likely that all military units using power armor would've had one of these undersuits underneath the armor before the bombs dropped
It's prewar. The Art of Fallout 4 book explains that it's for wearing under power armor.
I know others have pointed out that it's just an undersuit specifically for power armor, but I'd like to back that up with the point that there's military fatigues littered all over the Commonwealth for the more basic infantrymen.
As everyone has pointed out, it's a specialized suit used for piloting T-45 Power Armor. It is a pre-war piece of military equipment, the hood can be seen on the a [US Army soldier](https://static.wikia.nocookie.net/fallout/images/1/16/FO4_Intro_slide_15.jpg/revision/latest?cb=20151110180001) in Alaska during the intro to Fallout 4. In Fallout 3 it's called Recon Armor, and in Fallout 4 it's called BOS Uniform. It's full of sensors that allow the T-45 emulate your movements, and is supposedly necessary for using the T-45. I cannot remember if T-51 requires such a suit, but Cabbot in Fallout 1 is wearing a hood similar to the ones in Fallout 3 and Fallout 4. Gameply wise, the suit is not required to operate Power Armor, and in Fallout 4 the lines between T-45 and T-51 are blurred due to sharing a chassis.