How do you decide where an aggregate boundary goes?
Answer it out loud before you open anything. The value of the flags below is in comparing them to what you actually said — including whether you named a cost, or only a principle.
The situation behind the question
An Order has line items, a shipping address, a payment record and a review. Loading an order currently loads all four. A report screen times out because it loads ten thousand orders to sum their totals.
What it is really testing
Whether the candidate reasons from invariants and transactional consistency rather than from object graphs. The classic mistake is drawing the aggregate around what looks related in a diagram, which produces a boundary that is both too large to load and too small to protect anything.