The spec for this action:
https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G11.2111850
An example of a form using this action:
https://www2.illinois.gov/rev/forms/reg/Documents/reg-1-mr.pdf
let structTree = null;
    try {
      structTree = this._readStructTreeRoot();
    } catch (ex) {
      if (ex instanceof MissingDataException) {
        throw ex;
      }
      warn("Unable read to structTreeRoot info.");
    }
    return shadow(this, "structTreeRoot", structTree);
  }
  /**
   * @private
   */
  _readStructTreeRoot() {
    const obj = this._catDict.get("StructTreeRoot");
    if (!isDict(obj)) {
      return null;
    }
    const root = new StructTreeRoot(obj);
    root.init();
    return root;
  }
  get toplevelPagesDict() {