# How to interpret the momentum structure figure of the inelastic neutron scattering data?

I am recently reading inelastic neutron scattering papers and I can always see figures like the one above (https://science.sciencemag.org/content/356/6342/1055). However, I couldn't understand what it means, especially don't understand the meaning of $$x$$ and $$y$$ axes. Could anyone help me please? Why the $$x$$ and $$y$$ axes can be written as $$[H,H,0]$$ and $$[K,-K,0]$$?