Any tag not defined in the Jelly tag library for Ant, but defined to the Ant namespace, is assumed to be an Ant task, target or datatype. When Jelly tries to execute this unknown Ant tag, it looks for a real Ant element to match the tag name, and if found, matches the tag's attributes to the Ant element's attributes, as Ant would.
You never code this tag directly, instead you use it by defining the ant tag library namespace and the using an Ant element. For example, to use the Ant copy task you would code something similar to this:
<jelly xmlns:ant="jelly:ant"> <ant:copy file="myfile.txt" tofile="mycopy.txt"/> <jelly>