Every node has an Immune Flag, when on and the node is inside a Clone, it is not affected by any change to the clone, so you can store extra data in the clone or set a node to be ignored by the cloning process. Components also allow you to set Immune for the contents inside.
You can see if a node is Immune by inspecting the Immune Flag.
What is kept immune: the parameters, the wiring coming to the inputs of the node, and, if the node is a panel, all its Panel Values like
v. The data of Table DATs and the data of Locked nodes are also kept immune.
WARNING: If you add a new node in a Clone and make the new node's Immune flag On, you may also have to make the Immune flag On of the nodes its output is connected to, since wiring information is held with the inputs of a node.
Immune on Components
Immune by Python
cloneImmune in OP_Class.