diff --git a/share/mk/bsd.opts.mk b/share/mk/bsd.opts.mk
--- a/share/mk/bsd.opts.mk
+++ b/share/mk/bsd.opts.mk
@@ -1,6 +1,9 @@
 # $FreeBSD$
 #
-# Option file for src builds.
+# Option file for bmake builds. These options are available to all users of
+# bmake (including the source tree userland and kernel builds). They generally
+# control how binaries are made, shared vs dynamic, etc and some general options
+# relevant for all build environments.
 #
 # Users define WITH_FOO and WITHOUT_FOO on the command line or in /etc/src.conf
 # and /etc/make.conf files. These translate in the build system to MK_FOO={yes,no}
diff --git a/share/mk/src.opts.mk b/share/mk/src.opts.mk
--- a/share/mk/src.opts.mk
+++ b/share/mk/src.opts.mk
@@ -1,6 +1,8 @@
 # $FreeBSD$
 #
-# Option file for FreeBSD /usr/src builds.
+# Option file for FreeBSD /usr/src builds, at least the userland and boot loader
+# portions of the tree. These options generally chose what parts of the tree to
+# include or omit and are FreeBSD source tree specific.
 #
 # Users define WITH_FOO and WITHOUT_FOO on the command line or in /etc/src.conf
 # and /etc/make.conf files. These translate in the build system to MK_FOO={yes,no}
diff --git a/sys/conf/kern.opts.mk b/sys/conf/kern.opts.mk
--- a/sys/conf/kern.opts.mk
+++ b/sys/conf/kern.opts.mk
@@ -1,6 +1,9 @@
 # $FreeBSD$
 
-# Options set in the build system that affect the kernel somehow.
+# Options set in the build system which affect the building of kernel
+# modules. These select which parts to compile in or out (eg INET) or which
+# parts to omit (eg CDDL or SOURCELESS_HOST). Some of these will cause
+# config.mk to define symbols in various opt_*.h files.
 
 #
 # Define MK_* variables (which are either "yes" or "no") for users