When using equery -u to display the USE flags, with some packages the description for the USE flags is missing and displayed as unknown. E.g for mariadb:
farout ~ # equery u mariadb [ Legend : U - final flag setting for installation] [ : I - package is installed with flag ] [ Colors : set, unset ] * Found these USE flags for dev-db/mariadb-10.2.14: U I + + backup : <unknown> - - bindist : Flag to enable or disable options for prebuilt (GRP) packages (eg. due to licensing issues) - - client-libs : <unknown> + + cracklib : Support for cracklib strong password checking - - debug : Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces + + extraengine : <unknown> - - galera : <unknown> - - innodb-lz4 : <unknown> - - innodb-lzo : <unknown> - - innodb-snappy : <unknown> - - jdbc : <unknown> - - jemalloc : <unknown> - - kerberos : Add kerberos support - - latin1 : <unknown> - - libressl : Use dev-libs/libressl as SSL provider (might need ssl USE flag), packages should not depend on this USE flag - - mroonga : <unknown> - - numa : <unknown> - - odbc : Add ODBC Support (Open DataBase Connectivity) - - oqgraph : <unknown> - - pam : Add support for PAM (Pluggable Authentication Modules) - DANGEROUS to arbitrarily flip + + perl : Add optional support/bindings for the Perl language - - profiling : <unknown> - - rocksdb : <unknown> + + server : <unknown> - - sphinx : <unknown> - - sst-mariabackup : <unknown> - - sst-rsync : <unknown> - - sst-xtrabackup : <unknown> - - static : !!do not set this during bootstrap!! Causes binaries to be statically linked instead of dynamically - - static-libs : Build static versions of dynamic libraries as well - - systemd : Enable use of systemd-specific libraries and features like socket activation or session tracking - - systemtap : <unknown> - - tcmalloc : Use the dev-util/google-perftools libraries to replace the malloc() implementation with a possibly faster one - - test : Workaround to pull in packages needed to run with FEATURES=test. Portage-2.1.2 handles this internally, so don't set it in make.conf/package.use anymore - - tokudb : <unknown> + + xml : Add support for XML files - - yassl : <unknown>