What's the difference between 'unfulfilled' and 'fulfillment in progress' in order status?

Hi, I was wondering what the difference is between Unfulfilled and Fulfillment in progress in the order page is. I have two skus that are marked unfulfilled while 4 skus are marked Fulfillment in progress in the same order. Is there a way to group these together or why is it that these are separated? Thanks

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