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Can anyone let me know what this relates to?
I am guesing it is an adwords account setting of some description or even analytics related. It is just puzzling beacuse I see the majority of dest urls with it but some without.
If yes, then Google will add its own parameters in the destination URLs for tracking in Google analytics.
This option is present under the "My account" tab as "Tracking" and "Auto Tagging" will be enabled.