For an ATM banking project the following applies: "When the user first inserts a valid...

State transition testing is a technique that models the behavior of a system as a finite number of states and the transitions between them. It is suitable for testing systems that have different outcomes depending on the previous history of inputs and events. In this case, the ATM system has different states depending on whether the user has entered a valid or invalid PIN code, and how many times they have tried. 1-switch coverage is a criterion that requires testing all possible pairs of transitions from one state to another. This can help to find failures that occur when the system changes from one state to another, or when the system does not change state when it should. For example, 1-switch coverage would test the following scenarios:

User enters a valid PIN code on the first try and requests a cash advance (valid transition from unauthenticated to authenticated state)

User enters an invalid PIN code on the first try and then a valid PIN code on the second try and requests a cash advance (valid transition from unauthenticated to authenticated state after one invalid attempt)

User enters an invalid PIN code on the first and second try and then a valid PIN code on the third try and requests a cash advance (valid transition from unauthenticated to authenticated state after two invalid attempts)

User enters an invalid PIN code on the first, second, and third try and the system rejects the card (valid transition from unauthenticated to rejected state after three invalid attempts)

User enters a valid PIN code on the first try and then requests a cash advance twice (invalid transition from authenticated to authenticated state without inserting the card again)

User enters a valid PIN code on the first try and then removes the card and inserts it again and requests a cash advance (invalid transition from authenticated to unauthenticated state without entering the PIN code again)

References = ISTQB Advanced Level Test Analyst Syllabus 2019, section 3.2.3; How to Design Test Cases Using State Transition Testing Technique?; 0 and 1 - Switch Coverage in State Transition Testing?