there are two parts to the expansion: Add-In and Add-On. Add-in is a dungeon (Watcher's keep) which is accessable from both, SoA and ToB. The Add-on is the areas that you can access after finishing SoA. The good think about installing ToB before finishing SoA is that it raises the XP cap up to 8 mln. So, I would recommend getting it ASAP if you are planning to get it anyway.
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