This forces NX to update all of the external references. For instance, there are functions in the expression system that let you reference an external spreadsheet. When the model is opened, NX checks the spreadsheet values and updates the expressions accordingly. After the model is open, NX does not sense when changes are made to the external spreadsheet. If you want to update the spreadsheet and see the effect in your model, you'll need to "update for external change". There are other use cases, but linking expression values to a spreadsheet is probably the most common.
