systemd/man/user-system-options.xml

<?xml version="1.0"?>
<!DOCTYPE variablelist PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<variablelist>
  <varlistentry id='user'>
    <term><option>--user</option></term>

    <listitem id='user-text'>
      <para>Talk to the service manager of the calling user,
      rather than the service manager of the system.</para>
    </listitem>
  </varlistentry>

  <varlistentry id='system'>
    <term><option>--system</option></term>

    <listitem id='system-text'>
      <para>Talk to the service manager of the system. This is the
      implied default.</para>
    </listitem>
  </varlistentry>

  <varlistentry id='host'>
    <term><option>-H</option></term>
    <term><option>--host=</option></term>

    <listitem id='host-text'>
      <para>Execute the operation remotely. Specify a hostname, or a
      username and hostname separated by <literal>@</literal>, to
      connect to. The hostname may optionally be suffixed by a
      port ssh is listening on, separated by <literal>:</literal>, and then a
      container name, separated by <literal>/</literal>, which
      connects directly to a specific container on the specified
      host. This will use SSH to talk to the remote machine manager
      instance. Container names may be enumerated with
      <command>machinectl -H
      <replaceable>HOST</replaceable></command>. Put IPv6 addresses in brackets.</para>
    </listitem>
  </varlistentry>

  <varlistentry id='machine'>
    <term><option>-M</option></term>
    <term><option>--machine=</option></term>

    <listitem id='machine-text'>
      <para>Execute operation on a local container. Specify a container name to connect to, optionally
      prefixed by a user name to connect as and a separating <literal>@</literal> character. If the special
      string <literal>.host</literal> is used in place of the container name, a connection to the local
      system is made (which is useful to connect to a specific user's user bus: <literal>--user
      --machine=lennart@.host</literal>). If the <literal>@</literal> syntax is not used, the connection is
      made as root user. If the <literal>@</literal> syntax is used either the left hand side or the right hand
      side may be omitted (but not both) in which case the local user name and <literal>.host</literal> are
      implied.</para>
    </listitem>
  </varlistentry>
</variablelist>