How Special/Field Tags Affect Link Tracking

Links in an email may include a special tag or field tag. A common example is a user id or email address appended to a link as a parameter: For example, in the link below, custid might be a field in your account and %%custid%% is the corresponding field tag.
http://mysite.com/foo/bar?sale=3322fe&custId=%%custid%%

Below are some examples of links which contain tags.

<a href="http://mysite.com/foo/bar?sale=3322fe&custId=%%custid%%">My Site</a>
<a href="http://mysite.com/foo/%%sometag%%">My Site</a>
<a href="%%sometag%%">My Site</a>