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._buttonCaption) {
      return this._buttonCaption[nFace];
    }
    return "";
  }
  buttonGetIcon(nFace = 0) {
    if (this._buttonIcon) {
      return this._buttonIcon[nFace];
    }
    return null;
  }
  buttonImportIcon(cPath = null, nPave = 0) {
    
  }
  buttonSetCaption(cCaption, nFace = 0) {
    if (!this._buttonCaption) {
      this._buttonCaption = ["", "", ""];
    }
    this._buttonCaption[nFace] = cCaption;
    
    
    
    
  }
  buttonSetIcon(oIcon, nFace = 0) {