What is a null plane? So the OED gives a physics definition;

a plane in a complexified space–time, whose real slice is a null line.

And there is also this paper ("Null plane invariance of null plane dynamics" by F.Coester, W.H.Klink and W.N.Polyzou) which seems to give some mathematical definition. Is there an intuitive definition of a null plane?
The context that I am reading this in is; QCD and other field theories on the light cone by S. Brodsky, H-C Pauli, S. Pinsky, Section 3.
 A: The null planes are defined by normal vectors that point in a null direction.
Relativity
In special and general relativity null is used as a synonym for light-like when talking about spacetime.  Null is the preferred term in the language of differential geometry.
A light-like or null vector has zero magnitude.  Its inner product with itself is zero:
$$\vec{a}\cdot\vec{a} = a^\alpha g_{\alpha\beta} a^\beta = 0$$
Light travels along null geodesics. At every point along the curve the tangent vector points in a null direction.
A light cone is an example of a null surface. Every point on the light cone has zero spacetime separation from every other point, i.e. $\Delta\vec{x}\cdot\Delta\vec{x} = 0$ for any two points on the surface.
That Paper
That paper defines null planes as follows:

... it is useful to consider the manifold of null
planes $n\cdot x = x^0 + \hat{n}\cdot{x} = 0$ parameterized by the unit vectors $\hat{n}$...

Later we see $n$ defined more fully:

The essential mathematical feature is the coset decomposition $\Lambda = \mathcal{R}_H\Lambda_H$, $\Lambda_H\in H$ for all Lorentz transformations $\Lambda$, where $\Lambda_H$ leaves the null-plane $z\cdot x = 0$ invariant and the rotations $\mathcal{R}_H$ are parametrized by the components
of the spatial unit vector $\hat{n}, n := \{-1, \hat{n}\} = \mathcal{R}(n)\{-1,0,0,1\}$.

The paper doesn't use arrows to denote 4-vectors, but it seems that the 4-vector $n$ is a null vector.  Since it's spacial part is a unit vector $\hat{n}$, $n\cdot n = 0$.
It seems a null plane is defined by a null vector $n$ as the set of all $x$'s where $n\cdot x = 0$.
A: A null plane in $\mathbf{C}^n$ with respect the quadratic form $(dz^1)^2 + \cdots +(dz^n)^2$ is a plane, with the property that the quadratic form restricted to it  is degenerate.
A: Null  planes  are the  planes  tangent to the light cone.
