Datamerge allows you to dynamically add Extended Dataonto your responses. In other words, it will add them to your response rather than substituting into your response as Subtags would. Note that datamerge only works with Extended Data (not the entire Enhanced Data set).
For example, assuming you had an Extended Data field set up as 'Colour' and the data for that field for a particular Tag Group was 'Blue'. You could use datamerge to change your Redirect URL from :