Core-excised [0.5–2] keV luminosity (upper panel) and gas fraction (bottom panel) within r₅₀₀ vs. M₅₀₀ color-coded by brightness within 300 kpc for the XUCS sample. The solid black line and the dashed corridor indicate the mean relation and the ±1σ_intr regions derived in Papers I and II. Low surface brightness clusters (with μ₃₀₀ < 43.35 erg s⁻¹ Mpc⁻²) are indicated by squares and are mostly clusters that are X-ray faint for their mass and have a low gas fraction (near or below the lower dashed line) with some contamination by objects with an average gas fraction that are faint because the mass is low (near the solid line). We plot the measurements from two different telescopes for three telescopes, although it can be challenging to identify these duplicates in the plot.