We can certainly find some examples where the logic of whether a data hierarchy should only be present based on the value of another element. Classic example:
items
+--- tobacco user: Y/N
+--- tobacco use details
+--- amount per week
+--- cessation attempts
| +--- xxxx
+--- yyy
rules
/items[tobacco user] = 'Y' implies exists /items[tobacco use details]
If we consider the above rule a semantic property of the data independent of whether it is on the screen, in a message, document, survey or whatever other form, agreed on by the archetype experts, then it belongs within models - either the archetype or somewhere similar. (@erik.sundvall 's case)
However we can undoubtedly find examples where the same structural conditionality is use / context specific, and not applicable to all instantiations of the template as data. We may well need a UI level of mechanism to accommodate this. (@pablo 's case)
So it just means that not everything that looks like a certain kind of business rule for data or UI is necessarily one or the other.