I've heard the term seamfulness in design before without properly grasping it and it took me some time to flesh out. The basic gist as I understand it is that our experience of the "real world" is characterized by intrinsic knowledge of limitations (seams) - but knowledge thereof allows for intuitive navigation of said seams - knowing where the edges/seams are gives us a more satisfying/richer experience than running headlong into less-understood limitations where the seams have been hidden from us (ironically for our own convenience).

From the article "when we know where the seams are and how to manipulate them. The power in being able to manipulate the seams creates an elevated experience, one that emulates the ease of a seamless experience with the added power of control. Where seams help you identify how any one element interacts with the broader environment, the knowledge of ‘beautiful seams’ (source) could become invaluable to the user experience."