PNG Compression Level

I noticed in the Photoshop Scripting guide, PNGSaveOptions has a member called "compression", which takes an integer between 0 and 9, inclusive.

However, I am confused as to what this value corresponds to. In the GUI, when saving a PNG, the only options are these:

I'm not sure how a value between 0 and 9 corresponds to "None/Fast" or "Smallest / Slow".

Can somebody please explain what each of the 0-9 compression levels correspond to, or how they work? I would appreciate some background. Thanks.

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Legend

"None/Fast" -> compression=0

"Smallest/Slow" -> compression=9

Inspiring

So then what's everything in between? (1-8)?

Inspiring

Does anybody know what the levels in between correspond to? I have still been unable to locate this information.

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