How do the Usage Limits work?
The usage limit section applies at the individual code level, whereas the other settings apply at the discount level (over all codes within that discount.)
"Limit number of times each code can be used in total" will allow each code within the set to be used a set amount of times. We typically recommend setting this to 1.
"Limit to one use per customer" limits usage based on a customer's email address. We typically recommend checking this box.