What unit is Rigidbody Mass based on?

Is there specific unity mass is measured in in unity? Or is it simply relative to other objects in the scene?

I am creating a top-down car game, and I want to know what I should set the mass of my car to.

Thanks!

The weight of the object in kilograms.
It is recommended to make masses not
more or less than 100 times that of
other Rigidbodies.

I am not really sure about this, but I think that 1 unit is 1000 kilos.
Why?
I was trying to make a car and there was NO WAY to set up good joints, everything was falling apart and all the behaviour wasn’t really good.
So I had the illumination, I tried it and it works.
If you wonder why your physics buildings fall apart or have weak joints, it’s because of this.

Yeah, it’s in kilograms. Unity - Manual: Rigidbody component reference