systemd/man/user-system-options.xml

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<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
      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>.</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.</para>
    </listitem>
  </varlistentry>
</variablelist>