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Seed descriptions?

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I'm a bit confused about part of the seed descriptions of one of our sponsors?? They have seeds described like this: "Critical Mass (A) 5F/7F" & "Cheese 5F (A)" I have no idea what the 5F and 7F means?? I'm guessing the (A) means autoflowering since that's the section I was in.... but I need help with the "F" stuff. Thanks! :peace: