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) {
      return;
    }
    if (!Array.isArray(indices)) {
      indices = [indices];
    }
    if (
      !indices.every(
        i =>
          typeof i === "number" &&
          Number.isInteger(i) &&
          i >= 0 &&
          i < this.numItems
      )
    ) {
      return;
    }
    indices.sort();
    if (this.multipleSelection) {
      this._currentValueIndices = indices;
      this._value = [];
      indices.forEach(i => {
        this._value.push(this._items[i].displayValue);
      });
    } else {
      if (indices.length > 0) {
        indices = indices.splice(1, indices.length - 1);