he EDD says about release_nodes_on_stageout:
"When set to 'true', this will do an equivalent of 'pbs_release_nodes -a' for releasing all the sister nodes when stageout operation begins"
So when PBS sees there's a stageout operation to be done, then it checks if 'release_nodes_on_stageout=true" and frees up the sister nodes.
I don't think we need to explicitly create a new error message if there's no stageout operation and yet release_on_stageout=true. I will document in the EDD that in this case, nothing happens, release_nodes_on_stageout is not consulted (ignored).