pkg: ALTABI - where is this variable documented?

[O. Hartmann]

I try to fetch freebsd:10:x86:64 binary packages from a CURRENT host for
further processing. I got a hint using pkg with -o ALTABI=... -o ABI=... set to
the specific ABI. I try to find some explanations what I'm doing by studying
the documentation of pkg, pkg.conf, pkg-repo, and pkg-fetch. But I'm unable to
find any documentation/explanation about ALTABI.

Can someone shed some light onto my clouded view? Is there a manpage where all
pkg-related variables are gathered an listed?

Thanks in advance and kind regards,

Oliver

P.S. Please CC me as I'm not subsribing this specific list. Thanks.