Haven't heard this term since the ferric tape days when it was enough level to overcome the hiss and noise floor of mics in a room.
There is self noise of the equipment itself. As in Microphone recording chain will have a self noise especially tube mics and preamps.
The sound in the recording space usually acoustically deadened to lower to self noise of the room. Outside rumble or from Air Con getting in the mic. Mics have a high pass to combat this.
The idea to lower the noise floor of the mic in a room when compressed to increase signal to tape would also raise
the noise floor. Thus many studios of the late 70's and 80's were completely lifeless and life was added by the new expensive digital FX overriding any god forbid room ambiance. LOL
The point was the room even now has to minimize outside noise from getting in (sound proofing)and how the sound in the room bounces around (acoustic treatment)
On the HDs the pre amps are the modern cheap but clean design. The self noise of the unit is excellent but add to it an unheralded guitar and the pickups pick up stray electromagnetic fields from the HDs. Electronics, Guitar Amps, TVs, computers, dimmer lights all induce electronic noise into the system and to minimize it you have to shield the guitar.
When I first got my HD the noise was unaccepable especially on high gain amps and FX until I shielded all my guitars.