Which external system must answer before you respond to the user?
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 asked who uses it before naming what you would build it with.
In the online store, checkout calls the payment provider, the inventory service, the email sender and the analytics collector. A slow email API took checkout down last week (illustrative). The interviewer asks: "Which of these is a boundary, which must answer before the user sees a confirmation, and how would you treat the rest?"
What it is really testing
Whether the candidate can discover boundaries and interfaces from responsibilities — which components must talk, where the system ends, and which dependency's answer the response actually needs — instead of treating every call as the same call.