If set, the build history scanned to see what nodes and workspaces were used, and then all such that are not currently in use are deleted.
If not set, the node labels are checked to see which nodes are able to run the build and any workspaces found (aside from the build doing the deletion) are deleted.

Originally this plugin only supported the node-label method, which meant that slaves that had never run the build (but could do) would each be contacted. Also, the node-label method does not take into consideration the possibility that there may be more than one build (of this job) running concurrently. The build-history method is safer and results in filesystem operations being sent to fewer nodes... but it does rely on there being a build history to inspect and will take progressively longer to calculate if the build history is excessively long.