"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
What do you mean by 'most subordinate'? Is it that there must be no parallel
entries at the same level in the hierarchy?
It's always better to provide a reference to the full text of a quoted text.
The hierarchy in this case is the the structural object classes hierarchy, not
the directory information tree (DIT). Read in RFC 4512 about what SUP in
object class description means:
Note that there's also an attribute type hierarchy defined with SUP.