If you're reading that page as a truth table, you can ignore that column. The rest of the table should read correctly.
In the application referenced on Page 4, the 74184 is being used as a 6-bit converter, as shown in the leftmost schematic on Page 3. In this application, the least significant bit of the 6-bit input is passed through to the output without being connected to the 74184. The values in the "Binary Words" column refer to the values of those 6-bit inputs; there are two for each row of the truth table because the low bit of the input isn't relevant.
In practice, you can ignore all of this. The 74184 was already obsolete when Practical Electronics for Inventors was written in 2000; today, it's positively ancient, and is not available through any reputable suppliers. The tasks which were once performed by this IC are typically implemented in a microcontroller now.
(For what it's worth, I doubt that many hardware manufacturers ever implemented the sort of deep 74184 cascading shown on pages 6-8 of that datasheet. The cost of the ICs involved, and the board area required for all of them, would have been prohibitive.)