What does it mean that relapse is part of extinction?

Occasional returns of the old behavior are normal and do not mean failure.

Example: You spent six weeks systematically extinguishing your habit of checking your phone first thing in the morning. You removed the charger from the nightstand, placed the phone in another room, and installed a replacement routine of five minutes of journaling before leaving the bedroom. It worked. For forty-two days, you did not touch your phone until after breakfast. Then you traveled for a work conference, slept in an unfamiliar hotel room with the phone on the nightstand because the alarm was set for an early flight, and on the first morning you were scrolling email before your eyes fully opened. The old behavior fired as if the last six weeks had never happened. You did not decide to check the phone. The context changed, and the original learning — the one you thought was gone — resurfaced automatically.

Try this: Review your current extinction target from earlier lessons in this phase. Write three specific scenarios in which the old behavior is most likely to resurface: one involving a context change (new environment, travel, disrupted routine), one involving re-exposure to the original reward (encountering the trigger stimulus unexpectedly), and one involving the passage of time (a period of low vigilance after weeks of success). For each scenario, write one sentence describing the lapse and one sentence describing how you will interpret it — not as failure, but as predicted Phase 3 behavior. Keep this document accessible so that when one of these scenarios occurs, you have a pre-written interpretation ready before the abstinence violation effect can take hold.