See the attached image.
This is a hypothetical building where there are two storeys (walls on lower storey are coloured green, on upper storey they are coloured blue). The transparent squares indicate floor levels (yellow) and roof levels (red).
The upper storey has a step in level though.
So you can see we end up with walls joining in L, T or X formation, but the walls that need to join each other are at various different levels.
Every time I come up against this kind of situation it g