The Java EE 5 Tutorial

Tag Handler Does Not Manipulate the Body

If the tag handler does not need to manipulate the body, the tag handler should implement the Tag interface. If the tag handler implements the Tag interface and the body of the tag needs to be evaluated, the doStartTag method must return EVAL_BODY_INCLUDE; otherwise it should return SKIP_BODY.

If a tag handler needs to iteratively evaluate the body, it should implement the IterationTag interface. The tag handler should return EVAL_BODY_AGAIN from the doAfterBody method if it determines that the body needs to be evaluated again.