I can answer that, since I made the heightmap. It's an important consideration in a large heightmap - and Beyond Skyrim's Tamriel would be 336 x 236 cells - if it were all in one worldspace.
We originally wanted to make Beyond Skyrim an extension of Tamriel in the original game worldspace but hit too many problems. Instead, I created the whole of Tamriel as a single worldspace, and we intended just to have transition points from Skyrim at the obvious places - the three gates and a couple of points on the coast.
Then we found the havok bugs that limit worldspaces to 128 cells square, so I separated BSTamriel into five worldsaces: Iliac Bay (High Rock and Hammerfell); Heartland (Cyrodiil, Elsweyr and Valenwood); Summerset Isles and Black Marsh. Each is 128 cells square, with an extra region around it for LOD. In the process I adjusted the heightmap to put in a few "choke points" to drive the player to a few specific transition points without having to resort to invisible walls over land. At sea we'll use the game border to keep the player in the right worldspace.
Edit: accidentally quoted only half BSTamriel's actual size: 336 x 236 cells