diff --git a/share/man/man9/Makefile b/share/man/man9/Makefile
--- a/share/man/man9/Makefile
+++ b/share/man/man9/Makefile
@@ -150,6 +150,7 @@
 	EVENTHANDLER.9 \
 	eventtimers.9 \
 	extattr.9 \
+	exterror.9 \
 	fail.9 \
 	fdt_pinctrl.9 \
 	fetch.9 \
diff --git a/share/man/man9/exterror.9 b/share/man/man9/exterror.9
new file mode 100644
--- /dev/null
+++ b/share/man/man9/exterror.9
@@ -0,0 +1,151 @@
+.\" SPDX-License-Identifier: BSD-2-Clause
+.\"
+.\" Copyright (c) 2025 The FreeBSD Foundation
+.\"
+.\" This documentation was written by
+.\" Mark Johnston <markj@FreeBSD.org> under sponsorship
+.\" from the FreeBSD Foundation.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (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 November 5, 2025
+.Dt EXTERROR 9
+.Os
+.Sh NAME
+.Nm exterror
+.Nd provide extended error information to userspace
+.Sh SYNOPSIS
+.Bd -literal -offset left -compact
+#define	EXTERR_CATEGORY EXTERR_CAT_MYCATEGORY
+.Ed
+.In sys/exterrvar.h
+.Ft int
+.Fn EXTERROR "int error" "const char *msg" ...
+.Sh DESCRIPTION
+The
+.Nm
+framework allows kernel to pass additional information on an error
+to userspace, besides the standard
+.Xr errno 3
+error code, which is too terse and lacking the context.
+.Pp
+It is especially visible with the commonly overloaded error codes like
+.Er EINVAL
+or
+.Er EIO ,
+which might occur at many places for the given syscall, or even
+outside the context of the current kernel call.
+Identifying the specific cause for the returned error only by
+.Va errno
+value practically requires guessing it.
+.Nm
+attaches additional data to the error itself, and records meta-data about
+the error place in the kernel source code.
+Intent is to make it easier for human user to identify the cause of error,
+not to augment the existing
+.Va errno
+mechanism for the programmatical consumption.
+.Sh USAGE
+Before
+.Nm
+can be used in the given source .c file, the category of extended errors
+should be allocated in the
+.In sys/exterr_cat.h
+file.
+The category is the unique integer, that would, together with the source
+line number, uniquely identify the extended error occurence.
+Then, the
+.Va EXTERR_CATEGORY
+symbol should be defined as an alias for the allocated category, as
+shown in the summary.
+.Pp
+Instead of typical code fragment to report an error
+.D1 return (EINVAL);
+code now can do
+.D1 return (EXTERROR(EINVAL, \[dq]Invalid length\[dq]));
+The error data and metadata is saved in the current thread storage.
+The metadata includes the category and source line number.
+.Pp
+Arguments to the
+.Fn EXTERROR
+macro:
+.Bl -dash
+.It
+First argument to
+.Fn EXTERROR
+is the errno error code.
+.It
+Second argument is the constant string with the unbound lifetime,
+which should tersely provide enough human-readable details about
+the error.
+.It
+The
+.Fn EXTERROR
+macro might take two optional 64bit integer arguments,
+which meaning is transparent for the subsystem.
+.El
+.Pp
+The strings passed as the second argument are only retained
+in the kernel text if the
+.Cd option EXTERR_STRINGS
+was enabled in the kernel config.
+.Pp
+The
+.Fn EXTERROR
+macro can be used in any context where current thread is defined.
+In other words, only interrupt contexts and context switch code
+formally cannot use EXTERROR.  For kernel threads, use of
+.Fn EXTERROR
+does not make sense because there is no userspace to retrive
+the extended error data.
+.Sh USERSPACE ACCESS TO EXTENDED ERROR DATA
+There is no syscall speed overhead for using
+.Nm
+for non-error case.
+In case of error that supplied the extended information,
+kernel copies out that information into the userspace
+per-thread area that was registered with it, typically on
+image activation, or later at the thread startup.
+The area is controlled by the
+.Xr exterrctl 2
+internal syscall, normally done by the userspace C runtime.
+.Pp
+Userspace programs do not need to access the extended information
+area directly.
+There is no field that is stable for the specific error condition.
+Instead, the base
+.Lb c
+functions
+.Xr err 3
+and
+.Xr warn 3
+were modified to print the extended information alongside with the
+.Va errno
+decoding, if available.
+.Sh SEE ALSO
+.Xr errno 3 ,
+.Xr err 3
+.Sh HISTORY
+The
+.Nm
+facility was introduced in
+.Fx 15.0 .