[tz] What's "right"?

[Paul Gilmartin]

Hello, TZ,

In: ftp://ftp.iana.org/tz/tz-link.html
I read:
    The tz code and data support leap seconds via an optional
    "right" configuration, ...

Empirically, this seems to assume time_t of TAI-10.  This isn't
GPS, which I understand to be TAI-19:
    http://www.bipm.org/utils/en/pdf/time_ann_rep/Time_annual_report_2015/25_utcgpsglo_TAR15.pdf

TAI-10 is (accidentally?) the epoch of the IBM z/OS TOD clock.

I find a couple references to "zoneinfo/right" in Makefile and NEWS.
Is there more complete documentation of "right" and its rationale
elsewhere?

Thanks,
gil