What is binary search on the answer space and how does it work?

Instead of searching within the input array, binary search on the answer space searches for the optimal value in a range of possible answers. Define the minimum and maximum possible answers, then binary search for the boundary where a feasibility check function transitions from possible to impossible. For example, "minimum capacity to ship within d days" has answers ranging from the maximum package weight to the total weight sum. For each candidate capacity at the midpoint, greedily check if shipping within d days is feasible. If feasible, try a smaller capacity; if not, try a larger one.