[Math] What does a vertical line through the top and middle portion of an equals sign mean

Question

I came across a new symbol in the paper Design-order, non-conformal low-Mach fluid algorithms using a hybrid CVFEM/DG approach by Stefan P. Domino. What does this symbol mean? It looks like it's being used as an equals sign here…

Here is the set of equations for context:

Answer 1

This looks like $\pm$ ("plus or minus") to me.