Where the link points. Required.
Where the traffic comes from, e.g. instagram, newsletter, google.
The channel type, e.g. social, email, cpc.
The campaign this link belongs to.
Mostly for paid keywords.
Tells two similar links apart, e.g. which button.
Your tagged URL
Tip: keep your source/medium/campaign naming consistent (all lowercase, no spaces) so your reports don't split one campaign into five.