The Script tag is not a control; during the page generation process, the dashboard's xsl filter creates javascript code automatically to support controls.

This javascript code is inserted in a script tag in the head tag of the page; the control support code is created in the same order the controls appear in the page.

By using acs:Script instead of a standard script tag, you are assured that your javascript code will be inserted in the dashboard's script tag exactly at the right place; for example, if you want to manipulate a HtmlEditor, place your code in an acs:Script tag below your HtmlEditor control, and you will be assured it will be available.