Yeah those are the horizontal spans I was referring to.
It’s a sorted list of X coordinates (left to right). If you group them in couples, they are begin/end intervals of pixels within region (visibles), but it’s actually more useful to manipulate them as a flat array, as I described.
I studied a bit the code and each scanline is prefixed by the Y coordinates, and uses an out of bounds terminator (32767).
It's a bit more than that. The list of X coordinates is cumulative - once an X coordinate has been marked as an inversion, it continues to be treated as an inversion on all Y coordinates below that, not just until the next Y coordinate shows up. (This manifests in the code as D3 never being reset within the NOTRECT loop.) This makes it easier to perform operations like taking the union of two disjoint regions - the sets of points are simply sorted and combined.
The source code describes it as "an unpacked array of sorted inversion points". If you can read 68k assembly, here's the implementation of PtInRgn:
https://github.com/historicalsource/supermario/blob/9dd3c4be...