Just out of curiousity I checked the INT AC 17 Template file: no two Building Materials have the same Intersection Priority Number.
Which is probably a good advice in general when creating Building Materials.
The fact is: when two intersecting skins have the same Intersection Priority Number, then there is a priority order among element types that goes like this from strongest to weakest:
Morph > Beam > Roof > Shell > Slab > Column > Wall
But if it is the same element type (such as tow Walls) and their Intersection Priority Numbers are the same, it is up to chance which will be stronger.
To avoid such possible cases it is advisable that no two Building Materials have the same Intersection Priority Number.
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