Instance Key is an additional parameter for the data set identification. It is not related to Data proper and allows creating different versions of the same model instance. 

Modify Model task inherits Instance Keys from the original model setup that is displayed on the Existing Instance Keys pane of the Instance Keys tab when you select a model for modification (Fig.). 

Instance Keys tab allows you to set up additional keys. The system WILL NOT allow you to modify the existing Instance keys. 

Please refer to the Data Model Editor section for more information about Instance Keys and their setup.