UML modelling semantics

Question

In today's lecture about Modelling techniques with respect to MDD using UML the lecturer stated that it's absolutely necessary to give a (possibly) textual description about the semantics of each diagram you produce.

In my opinion it's not necessary to describe semantics of diagram elements which are already standardized by UML. I can see that it's completely appropriate if you extend the standard UML by stereotypes/tagged values for any reason.

In contrast, I think that standardization has in the ordinary case the intention to let people reason and talk about diagrams without the need to explain semantics every time. The only precondition is of course that they all rely on the same UML specification.

Another point regarding MDD is that using different semantics from time to time for the same diagram kind makes code generation and automatic model transformation difficult in the end.

1. Am I right with this notion?
2. Are there some inherent interpretation ambiguities in UML to make it more general usable?