Final answer:
Box U - Retraite progressive on the RL-1 slip represents contributions made towards a progressive retirement plan in Quebec, Canada.
Step-by-step explanation:
'Box U - Retraite progressive' on the RL-1 slip refers to a retirement saving plan in Canada. The RL-1 slip is a statement of remuneration paid and deductions withheld for an employee. Box U specifically represents the amount of contributions made towards the employee's 'retraite progressive' or progressive retirement plan. It is important to note that 'retraite progressive' is a specific retirement plan available in Quebec, Canada.