A user defined node (UDN) is a node that can be created by the user.
It is a node that can be used in the same way as the built-in nodes, as well as unpacked.
UDN(user defined nodes)
One of the special qualities of level nodes is that a level node can be turned into a UDN, after which you can create a new node of this UDN type, and everything that's there when you defined the UDN will be created.
UDNs can store node graphs, parameters, and other types of metadata inside them.
A newly created UDN is by default packed, when it's packed, it behaves like one single node. This is useful if you do not wish to modify the contents of this UDN, some UDNs are created with the intention of generating different contents, of course most of the customisation should be available and set by the parameter, but if you wish to change the structure of the node graphs underneath, you can unpack the UDN.
Unpacked UDNs
Unpacked UDNs are in a state similar to how they were created, with all nodes available to be edited inside them. You can go inside an unpacked UDN and edit the contents to fit your needs, extend or remove the functionalities of the original UDN.
Be careful though, if you want to pack it again, you can do so by right clicking on the UDN and choose "Reload Defined", this reloads the UDN to the defined state, and your modification inside will be lost.
If you want to create a new UDN, you can do so by right clicking on the unpacked UDN and create a new one. If you are the owner of the original UDN or have permissions to edit them, you can update the definition of the UDN with the modified contents too.
When a UDN definition is being updated, all nodes created before will automatically pick up the new definition next time they run.