In The Elfstones of Shannara, the demons emerge from some point beyond the west boundary of the Westland, which is claimed by Allanon to be 'the weakest point in the Forbidding'.
In Jarka Ruus, it is revealed that The Forbidding is a world parallel to the main world of The Four Lands, like a mirror image.
In the Dark Legacy series, it is again somewhere beyond the Breakline mountains west of the Westland.
Is there any in-universe reference (Author's remarks, wikia, etc.) that explains these inconsistencies?