Back in the day, it meant that present versions of the Commodore BASIC and Kernel did not use those locations, but Commodore said nothing about whether future versions of the ROM might do so. Addresses 251-254, by contrast, were specifically marked as available for user programs and Commodore guaranteed that nothing in present nor future versions of the BASIC nor Kernel would do anything to conflict with such usage.
Of course, every version of the BASIC and Kernel ROMs that will ever be produced by Commodore, has been produced by now, and any locations which haven't been used by any versions released to date won't be used by any future versions, ever. Back in the day, however, programmers would have had no way of knowing that.
In official documentation, storage locations which aren't currently used, but which might be used in future, will often be marked "reserved". I don't remember whether Commodore documented the indicated addresses that way. Third-party documentation, especially written after the C64 was discontinued, will be more likely to regard unused locations as "unused" for the reasons indicated above.