Nodes have a nodeType based on an enum:
The character returned if the node is active
The character returned if the node is deleted
The character returned if the node type is unset/unknown.
A post request against /rest/nodes with a node definition that is not defining any nodetype is handled as a valid post. The node is created in the DB with nodeType = null. Nodes with nodeType = null are not visible or accessible in OpenNMS.