diff --git a/usr.bin/mkimg/mkimg.1 b/usr.bin/mkimg/mkimg.1
--- a/usr.bin/mkimg/mkimg.1
+++ b/usr.bin/mkimg/mkimg.1
@@ -22,7 +22,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd June 25, 2025
+.Dd December 30, 2025
 .Dt MKIMG 1
 .Os
 .Sh NAME
@@ -39,6 +39,7 @@
 .Op Fl C Ar max_capacity
 .Op Fl -capacity Ar capacity
 .Op Fl f Ar format
+.Op Fl h
 .Op Fl o Ar outfile
 .Op Fl a Ar active
 .Op Fl t Ar timestamp
@@ -144,6 +145,10 @@
 given time, specified in seconds since the epoch.
 .Pp
 The
+.Fl h
+option prints a usage message and exits.
+.Pp
+The
 .Fl v
 option increases the level of output that the
 .Nm
@@ -301,34 +306,43 @@
 A specification that is a single dash indicates an unused partition entry.
 Otherwise, a partition specification has the following format:
 .Bd -literal -offset indent
-<type> ':' <kind> <contents>
+<type>[/<label>]::<size>[:[+]<offset>]
+<type>[/<label>]:=<file>[:[+]<offset>]
+<type>[/<label>]:-<cmd>
+-
 .Ed
 .Bl -tag -width indent
 .It Cm type
 the partition type alias (f.e.: freebsd-swap)
-that may be optionally followed by a '/' separator
-and a label for partitioning schemes that feature partition labels
 (see the
 .Sx EXAMPLES
 Section below)
-.It Cm kind
-the interpretation of the contents specification:
-.Bl -tag -width indent
-.It Cm ':'
-contents holds the size of an empty partition,
-a number that may be suffixed with one of K, M, G, T, P or E
-(either upper or lower case) following the SI power of two convention
-(see also
+.It Cm label
+an optional label for partitioning schemes that support labels
+.It Cm size
+the size of an empty partition, a number that may be suffixed with one of
+K, M, G, T, P or E (either upper or lower case) following the SI power of
+two convention (see also
 .Xr expand_number 3 )
-.It Cm '='
-contents holds the name of a file to read
-.It Cm '-'
-contents holds a command to run; the output of which is the contents
-of the partition.
+.It Cm file
+the name of a file to read; the partition size is determined by the file
+.It Cm cmd
+a command to run; the output of which is the contents of the partition.
 Multi-word strings should be quoted according to the shell rules.
-.El
-.It Cm contents
-the specification of a partition's contents
+.It Cm offset
+an optional byte offset for the
+.Cm size
+and
+.Cm file
+forms.
+If prefixed with
+.Ql + ,
+it is relative to the end of the previous partition; otherwise it is an
+absolute offset from the start of the image.
+The value follows the same numeric conventions as
+.Cm size .
+.It Cm -
+an unused partition entry.
 .El
 .Sh ENVIRONMENT
 .Bl -tag -width "TMPDIR" -compact
diff --git a/usr.bin/mkimg/mkimg.c b/usr.bin/mkimg/mkimg.c
--- a/usr.bin/mkimg/mkimg.c
+++ b/usr.bin/mkimg/mkimg.c
@@ -152,15 +152,18 @@
 	fprintf(stderr, "\t--formats\t-  list image formats\n");
 	fprintf(stderr, "\t--schemes\t-  list partition schemes\n");
 	fprintf(stderr, "\t--version\t-  show version information\n");
+	fprintf(stderr, "\t--capacity\t-  minimum and maximum capacity (in bytes)\n");
 	fputc('\n', stderr);
 	fprintf(stderr, "\t-a <num>\t-  mark num'th partition as active\n");
 	fprintf(stderr, "\t-b <file>\t-  file containing boot code\n");
 	fprintf(stderr, "\t-c <num>\t-  minimum capacity (in bytes) of the disk\n");
 	fprintf(stderr, "\t-C <num>\t-  maximum capacity (in bytes) of the disk\n");
 	fprintf(stderr, "\t-f <format>\n");
+	fprintf(stderr, "\t-h\t\t-  show this usage information\n");
 	fprintf(stderr, "\t-o <file>\t-  file to write image into\n");
 	fprintf(stderr, "\t-p <partition>\n");
 	fprintf(stderr, "\t-s <scheme>\n");
+	fprintf(stderr, "\t-t <num>\t-  set timestamp (seconds since epoch)\n");
 	fprintf(stderr, "\t-v\t\t-  increase verbosity\n");
 	fprintf(stderr, "\t-y\t\t-  [developers] enable unit test\n");
 	fprintf(stderr, "\t-H <num>\t-  number of heads to simulate\n");