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
case "A":
        case "a":
          const LIMIT = 26; 
          const A_UPPER_CASE = 0x41,
            A_LOWER_CASE = 0x61;
          const baseCharCode = style === "a" ? A_LOWER_CASE : A_UPPER_CASE;
          const letterIndex = currentIndex - 1;
          const character = String.fromCharCode(
            baseCharCode + (letterIndex % LIMIT)
          );
          const charBuf = [];
          for (let j = 0, jj = (letterIndex / LIMIT) | 0; j <= jj; j++) {
            charBuf.push(character);
          }
          currentLabel = charBuf.join("");
          break;
        default:
          if (style) {
            throw new FormatError(
              `Invalid style "${style}" in PageLabel dictionary.`
            );
          }
          currentLabel = "";
      }
      pageLabels[i] = prefix + currentLabel;
      currentIndex++;
    }
    return pageLabels;
  }
  get pageLayout() {