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>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.</o>
    <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>
</script>