Thank you for you answer kabee.
It is not still clear for me why it is useful to have different properties for a placed part called instance or occurrence. The Capture user guide gives also a blurred explanation on the topic like "the information "shines through"from the cache to the instance and occurences of the part property". It is resonable that different parts from different projects have different properties and these differences should be handled somehow. But would it be more beneficial to decide at the placement of the copied part which property is required and which is not. On the other hand what indicates that you are in occurance or instance mode?
Could someone shed some ligth on it?