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
const source = this._objects[id];
    const event = (globalThis.event = new Event(baseEvent));
    let savedChange;
    if (source.obj._isButton()) {
      source.obj._id = id;
      event.value = source.obj._getExportValue(event.value);
      if (name === "Action") {
        source.obj._value = event.value;
      }
    }
    switch (name) {
      case "Keystroke":
        savedChange = {
          value: event.value,
          change: event.change,
          selStart: event.selStart,
          selEnd: event.selEnd,
        };
        break;
      case "Blur":
      case "Focus":
        Object.defineProperty(event, "value", {
          configurable: false,
          writable: false,
          enumerable: true,
          value: event.value,
        });
        break;
      case "Validate":
        this.runValidation(source, event);