A.
With the IATA cargo rates not being observed nowadays, in actual day-to-day market practice, the airline can apply any rate he wishes, whether above or lower than IATA levels. However, if you wish to be 100 percent legal, then, you need to follow the Hong Kong/Japan Bilateral Air Agreement which spells out that the rates between HK and JP must be the IATA rate. Any other rates will be considered “illegal” not following the stipulations of the HK/JP Bilateral Air Agreement. But, in reality, the market today doesn’t care. Both the HK and JP governments are not interfering. So as conclusion, “legally” IATA rates must be used, but daily market practice allow any rate, whether higher or lower that IATA levels.

For additional information, HK/US is “Open Sky Agreement” where any rate can be used as long as it is not a “dumping” rate. So, between HK and US, any rate is “legally” correct. So, if you want to be 100% legal, you need to follow the bilateral air agreement between HK and the destination country.