Are the Shards in separate dimension or are they physical locations?

[Iscah]

The way the shards have been spoken about before, they're all overlapping, just on different "planes", as if they're on different "frequencies" in the same "space"...but this may no longer be the working assumption.

You say that like there's a specific reason to reconsider that assumption - is there?

I don't remember there being any suggestion of other possibilities. Is there a reason to question it beyond "we don't know the technicalities of how it works in circumstances the story hasn't explored yet"?