I am unable to find any documentation for FLAVORS recently added to the ports/packages infrastructure from the end user point of view. Only the Porters Handbook contains some info. To add insult to injury, from /usr/ports/UPDATING two month ago: 20171130: AFFECTS: all ports users AUTHOR: adamw@FreeBSD.org [...] Flavors are already supported by make, pkg, poudriere, and synth; see the manpages for those tools for usage instructions. I am using first two tools from the list and guess what? I saw them and now I am writing this bug report :)
Document were added: https://www.freebsd.org/cgi/man.cgi?poudriere(8)#FLAVORS https://www.freebsd.org/cgi/man.cgi?ports#EXAMPLES