As noted by others starting and ending an xonex at different airports in Japan is legal under the rules and does not require the use of one of your 16 segments.
- (c) Travel may originate at any point for which fares are published and must terminate at the same point, except that origin‐destination surface segments are permitted as follows:
- within the country of origin
- within the Middle East
- between the United States and Canada
- between HKG and China
- between Malaysia and SIN
- within Africa
- between Maldives and Sri Lanka/India