Once an appointment is set by placing it on a block you are ready for patients to arrive. Upon arrival, in Open ACU you will check them in. This does two important things to your schedule and your patient's data. First it records a successful appointment, confirming the patient arrived. Second you can add transactional information to the appointment an therefore the patient.
Clicking the check icon at the far right of the appointment slot not only opens the transaction form modal, it records the fact that the patient showed up for their appointment. In your reports and in the patient record, plus on the schedule grid, it is clear which patients have shown up or not.
The check in also timestamps the checkin and the user who did so. All a part of the appointment log.
In Open ACU you cannot edit transactions, checkins or otherwise.