Transient/non-sticky mode - what is Hidden/auto-close

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A very minor question.

I've just decided (after some years) to refresh my knowledge of Wing (Pro6) by going through the latest tutorial.

got to "Transient, Sticky, and Locked Files - Wing opens files in one of several modes in order to keep more relevant files open, while auto-closing others. To see this in action, right-click on os in import os at the top of example1.py and select Goto Definition. The file os.py will be opened non-sticky, so that it is automatically closed when hidden."

my question - What defines 'hidden' (or auto-close in this context). Having opened a a module file R/O it stays as a tab in the editor until I close it in the same manner as other files(only with a horizontal pin!) .

What am I missing ??

ps . It's a good idea to redo the tutorial - I'd forgotten a lot of detail.