Only object's of a type which hold interface data (that is, types derived from mcobjTypeINTERFACE or mcobjTypeXOBJECT) will return a value other than Nothing. Furthermore, if the object is an alias without a ShadowMcObject, then Nothing is returned.
The property is exposed as Nothing if no interface data is available.
It is faster to use this method to access interface data than to get the Value property (which is a Variant). However, this is a read-only property, so if you wish to change the Interface property, you must assign to the Value property.
Interface Property
