Are Unicode blocks predefined, even before characters are encoded for them?
Formally, no. However, the Unicode Consortium and SC2/WG2 jointly maintain a Roadmap that contains both existing blocks and tentative allocations of blocks for future encoding. The tentative allocations help in the planning for encoding and provide a convenient place for linking to proposal documents. However, they are not part of the standard itself, and such tentative block allocations can be and frequently are moved around during the process of proposal review and approval. For details, see the Roadmaps.
