In a nutshell - a task slot is a resource management unit, where operator chains are executed in dedicated threads with state per the subtask. A slot obtains a portion of memory for the subtask execution and uses it mainly to keep states. A taskSkot has its own thread pool, which gets a chain of operators for execution. The chain is executed in a thread with exclusive access to its state. Usually, all operators of a chain of a graph (from source to sink) are executed in a thread and using resources of a task slot in the same TM but it is not obligatory.