Minecraft wiki entry for Updating_old_terrain, associated text: The first one is your base. The box of chunks that encloses your base has chunk (-1, 0, 4) in its upper left corner, and chunk (3, 0, 12) in its lower right corner. So, we write 'chunk_region(-1, 4, 3, 12),' to define this region.
The second one is a mob farm in the middle of the ocean. The entire farm fits into a single chunk, namely chunk (33, 0, 15). In this case, the upper left and lower right "corner" are the same chunk, so we write 'chunk_region(33, 15, 33, 15),'.