The single-month reimbursement is for a one-time expenditure on a reimbursable item. For an auto-renew membership, or for a multi-month membership with enough points to cover the reimbursement.
The multi-month reimbursement is only for those people who paid in advance (prepaid) and the membership is effective for more than one month. However, if your current account points are more than the total amount of the reimbursement, you should select single month for your reimbursement to get your reimbursement faster.
For example, suppose you bought the three-month gym membership for a total of $300 ($100 each month) and paid in advance on 01/14/19. Since you only have a $100 stipend each month, you cannot be reimbursed for the total gym dues ($300). In this case, you should submit for reimbursement with multi-month.
Or suppose you bought the four-month gym membership for a total of $100 ($25 each month) and paid in advance on 01/14/19. You have the $100 equivalent points, which can cover the total amount of your prepaid gym membership. In this case, you should choose a single month for reimbursement.