Emitting regions from which 15–85% of the ¹³CO J = 2–1 (grey solid box) and ¹²CO J = 2–1 (black solid box) radiation is emitted. Dotted lines indicate the emitting region in the vertical direction only. The grey and black crosses are mapped from the fitting functions to the respective emitting surfaces derived in Law et al. (2023). The white curve is the 1.5 × 10⁴ cm⁻³ contour for the critical density of the ¹²CO J = 2–1 transition with H₂ as the collision partner. The colour map is the gas density.