Structural object class rules

"An object or alias entry is characterized by precisely one structural object class superclass chain which has a single structural object class as the most subordinate object class. This structural object class is referred to as the structural object class of the entry."

There's a bit of ambiguity with this

"which has a single structural object class as the most subordinate object class"

What do you mean by 'most subordinate'? Is it that there must be no parallel entries at the same level in the hierarchy?