Example of "Using a Sempahore to regulate access" in DU Process Template

I’m learning how to use the Document Understanding Process template, and there is a comment within the 40_TrainClassifiers workflow that says 1 of the types of synchronization mechanisms available is “Using a Sempahore to regulate access.” You can see it in context below.

It says to “see the Examples folder for sample implementations.” There is no Examples folder.

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Anyone know anything about this “semaphore” synchronization mechanism that is “readily” available??