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
if (!this._isChoice) {
      throw new Error("Not a choice widget");
    }
    if (nIdx < 0 || nIdx >= this.numItems) {
      nIdx = this.numItems - 1;
    }
    const item = this._items[nIdx];
    return bExportValue ? item.exportValue : item.displayValue;
  }
  getArray() {
    if (this._kidIds) {
      return this._kidIds.map(id => this._appObjects[id].wrapped);
    }
    if (this._children === null) {
      this._children = this._document.obj._getChildren(this._fieldPath);
    }
    return this._children;
  }
  getLock() {
    return undefined;
  }
  isBoxChecked(nWidget) {
    return false;
  }
  isDefaultChecked(nWidget) {
    return false;
  }
  insertItemAt(cName, cExport = undefined, nIdx = 0) {