node-red/packages/node_modules/@node-red/nodes/locales/en-US/common/60-link.html

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<script type="text/html" data-help-name="link in">
    <p>Create virtual wires between flows.</p>
    <h3>Details</h3>
    <p>The node can be connected to any <code>link out</code> node that exists on any tab.
       Once connected, they behave as if they were wired together.</p>
    <p>The wires between link nodes are only displayed when a link node is selected.
       If there are any wires to other tabs, a virtual node is shown that can be clicked
       on to jump to the appropriate tab.</p>
    <p><b>Note: </b>Links cannot be created going into, or out of, a subflow.</p>
</script>

<script type="text/html" data-help-name="link out">
    <p>Create virtual wires between flows.</p>
    <h3>Details</h3>
    <p>This node can be configured to either send messages to all <code>link in</code>
       nodes it is connected to, or to send a response back to the <code>link call</code>
       node that triggered the flow.</p>
    <p>When in 'send to all' mode, the wires between link nodes are only displayed when
       the node is selected. If there are any wires to other tabs, a virtual node
       is shown that can be clicked on to jump to the appropriate tab.</p>
    <p><b>Note: </b>Links cannot be created going into, or out of, a subflow.</p>
</script>

<script type="text/html" data-help-name="link call">
    <p>Calls a flow that starts with a <code>link in</code> node and passes on the response.</p>
    <h3>Inputs</h3>
    <dl class="message-properties">
      <dt class="optional">target<span class="property-type">string</span></dt>
      <dd>When the option <b>Link Type</b> is set to "Dynamic target", set <code>msg.target</code> to the name of the
          <code>link in</code> node you wish to call.</dd>
    </dl>
    <h3>Details</h3>
    <p>This node can be connected to a <code>link in</code> node that exists on any tab.
       The flow connected to that node must end with a <code>link out</code> node configured
       in 'return' mode.</p>
    <p>When this node receives a message, it is passed to the connected <code>link in</code> node.
       It then waits for a response which it then sends on.</p>
    <p>If no response is received within the configured timeout, default 30 seconds, the node
       will log an error that can be caught using the <code>catch</code> node.</p>
    <p>When the option <b>Link Type</b> is set to "Dynamic target" <code>msg.target</code> can be used to call a
       <code>link in</code> by name or id. 
    <ul>
      <li>If there is a <code>link in</code> nodes with the same id, it will be called</li>
      <li>If there are two or more <code>link in</code> nodes with the same name, an error will be raised</li>
      <li>A <code>link call</code> cannot call a <code>link in</code> node inside a subflow</li>
    </ul>
    </p>
    The flow connected to that node must end with a <code>link out</code> node configured
    in 'return' mode.</p>
</script>