This button pops-up a menu:

Add Candidate: Open a candidate selection and add the candidate to the shift.
Delete Shift: Delete this shift and close this dialogue.
Mark as Checked In: This will flag that the candidate has safely arrived at the work address for this shift.
Mark as Assigned: This will change the booking status from confirmed to assigned.
Mark as Confirmed: This will change the booking status from assigned to confirmed.
Remove Due to No-Show: This will remove the candidate from the booking and is used when they have failed to arrive for work.
Remove: This removes the candidate from the shift.