I'm assuming that is not what you are doing. What you should be using is UID, which you can get from object.uid. This can be stored in an array or variable when you create the object (pushing it to an array will give you the order each one is created in as well).
Otherwise, the shortcut to use IID is to reference the newest instance of an object in an expression by object(-1).expression.
You generally never need to know to know what the IID of any specific object is, as it can change as objects are destroyed.
So IID basically is a way to get relative order for expressions - first, second, third, last, next to last, ect. UID useful to pick a specific instance of an object.