I don’t think there is documentation specifically on this. These are the field key reference to each field. This is how ACF knows what field the value belongs to so it knows how to handle that value. There is a paragraph explain it on this page https://www.advancedcustomfields.com/resources/update_field/
How you create this value depends on how you are migrating the database. All meta values should be copied with importing.
A bigger problem that you’re going to have is any field that is storing an ID value, for example, image, gallery, post object and relationship field all store post IDs of the related object. Most imports create new IDs for posts and this means that all of the existing values will be wrong.
