It's the same one as implemented in MS Word (and PowerPoint, OneNote), this standard was written by an engineer who is on the Math in Office team and maintains a blog [1]. It's the best WYSIWYG equation editor I've used so far. LaTeX has its place, but it's hard to convince non-coders to use it and I'm glad it has the editor. I stumbled into this specification trying to format a constrained optimization problem and found the equation array format to be fairly straightforward similar to the bmatrix environment in LaTeX and thought it was neat that Unicode could represent so much math.