I'm not sure what these terms mean. In my lecture notes I wrote that grammatical case is used to show the syntactic functions of a nominal syntagm, depending on its relation to the verb. Semantic case, on the other hand, doesn't show the syntactic function but rather the semantic one.
As I don't have any examples for this, I find it hard to understand. Could this for example mean that even though a certain nominal syntagm is grammaticaly in accusative (so there are affixes for accusative) it actually has attributive meaning (so semantically it's genitive)?