PSEUDO_FLAVORS

PSEUDO_FLAVORS allow you to control the build process a bit.  Usually they are used to prevent building a specific subpackage.  In mail/dovecot, the no_sieve PSEUDO_FLAVOR skips packing the -sieve subpackage:

...
COMMENT-sieve=  sieve mail filtering for Dovecot
...
PSEUDO_FLAVORS= no_sieve
...

.if !${FLAVOR:L:Mno_sieve}
MULTI_PACKAGES+=   -sieve
SIEVE_DIR=         ${WRKDIR}/dovecot-sieve-${V_SIEVE}
AUTOCONF_DIR+=     ${SIEVE_DIR}
RUN_DEPENDS-sieve= ::${BUILD_PKGPATH}
LIB_DEPENDS-sieve= ${MODLIBICONV_LIB_DEPENDS}
.endif

...