This example shows it in the simplest form grassland inside a forest. The grassland is a simple polygon, but if you couple this to a second grassland patch they together become a multipolygon (MP) where both are an outer member of their MP. When one is inside the forest, then only the 1 is made also an inner member of the Forest. Then you get the Forest being the outer in the MP relation and the grassland patch the inner

https://www.openstreetmap.org/relation/282825#map=14/45.8261/6.6311

If you click on the outer way, only the forest is highlighted. If you click on the inner way, only the grassland is highlighted. You see it's in first order tagged as grassland, but then made an inner member of the forest MP.

In your case the ski zones are just being outlined on top of everything else so they're not defined as 'inner' to anything i.e. this https://www.openstreetmap.org/relation/17261841 being a 3 part outer MP relation.