A VI that is non-reentrant means there is only ever a single copy of the VI that is running at any given time. If you try to call a non-reentrant VI from multiple places at the same time, each subVI instance has to wait until the other subVI instances have stopped using it. With a reentrant VI, a copy of the VI is made which called 'clone VI'. That means more than one copy of the same subVI can run at the same time, each with its own memory space.
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