They are actually stored as serialized data. ACF holds these values as arrays and when WP saves these values to the DB it converts them to serialized data. This is how WP stores all objects and arrays.
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ACF wouldn’t be so widely used in WordPress if it didn’t have some pretty amazing capabilities. In this article, we look at a few of the features we’ll discuss during “7 things you didn’t know you could do with ACF” at #WPEDecode later this month. https://t.co/5lnsTxp81jpic.twitter.com/Yf0ThPG1QG