The Rules of Evidence
Whether a SANE testifies as a fact witness, an expert witness, or as both, her testimony is limited by the rules of evidence specific to her state or, in federal court, by the federal rules of evidence, which are the basis for most state rules.
As explained in Module 2, the rules of evidence are very strict and impose significant limitations on what witnesses of any kind may say. Sometimes these rules do not seem to make sense, but witnesses must abide by them nonetheless.