When batch tasks get invoked at the completion of a build (that is when the "Invoke batch tasks" post-build action is used), then each batch task uses the latest build rather than the build that invoked the task.

For example, doing the following:
1) Create a project and turn on concurrent building
2) Have the project sleep as part of the main build in order to have multiple builds occur at the same time
3) Add a batch task that echos the workspace and invoke it as a post-build action.
4) Start 2 builds such that the second starts before the first completes

In the console output of Build #1, it will show that it is invoking task 2-1 and Build #2 invokes task 2-2.
Both tasks use Build #2's workspace as denoted by the naming convention 2-#.

The expected behavior is for Build #1 to invoke task 1-1 with Build #1's workspace, and Build #2 to invoke task 2-1 with Builds #2's workspace.

This allows concurrent building to invoke batch tasks such as cleaning a workspace as a post-build action.