Top left: probability distribution functions (PDFs) of the integrated intensity emission for the full integrated intensity map (black), within the spiral arms (red), for inter-arm regions (blue), and towards the Galactic centre (green). Top middle: PDFs normalised by the total counts (N) in the distribution. Top right: relative value of the normalised PDFs of each region with respect to the normalisedtotal distribution (named ‘All’ in the left panel). In the top-left panel legend, IDI median (μ) and median absolute deviation (σ) are shown for each region (where SA, IA, and GC stand for spiral arms, inter-arm region, and Galactic centre, respectively). The IDI thresholds are indicated with grey vertical dotted lines. Bottom row: parallel distribution representations using CO luminosity from clouds. Statistics for the LDI from each considered region are indicated in the bottom-left panel legend.