The Use This Cookie Crumb check box causes the tag to extract a cookie crumb from the cookie specified above. Cookie crumbs are key/value pairs contained within a cookie value. Typically these pairs are separated by a special character known as a separator. The name of the crumb is specified in the accompanying text box. The crumb value is then assigned to the query parameter specified in the accompanying text box.
More information here.
Suppose you had a cookie named MYCOOKIE with the following value: MYCOOKIE=alpha=1&beta=2&gamma=3
MYCOOKIE=alpha=1&beta=2&gamma=3
In this case, the crumb names are: alpha, beta, gamma. The crumb values are: 1, 2, 3.
