This linear regression model may be considered as the combination of two pieces of knowledge (see later), one is specified by the linear equation involving three variables X, Y, and E, and the other is a simple normal distribution of E, i.e., E ~ N(0, Σ).
From representing the six special cases, we see a clear advantage of the moment matrix representation, i.e., it allows a unified representation for seemingly diverse types of knowledge, including linear equations, joint and conditional distributions, and ignorance.