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:
AFFECTS: all ports users
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 :)