The Instance ID (IID) of a SimCity 4 Object is the third set of eight hexadecimal alphanumeric characters making up the object's Type Group Instance (TGI) address. As an example, in the TGI "AAAAAAAA BBBBBBBB CCCCCCCC", the sequence "CCCCCCCC" is the object's IID.
An IID is an object's individual address within the object's type and group. For each type and group combination, IIDs provide 4,294,967,295 different discrete addresses, permitting the game to have a virtually unlimited number of objects that do not conflict with each other.
Instance ID's all follow a particular format, based on the Group they belong to. The list of these formats is here.