Clarification about Pro-Sites coupons

I've searched a lot of posts asking about info on pro-sites coupons and I still couldn't find a definite answer so I'm opening this issue:
Does the coupon apply to the first MONTH or the first PAYMENT?

If I were for example to create a coupon that subtracts the amount of the yearly plan and the user tries to apply this code when purchasing, will he get 12 months free?

Most posts here say it's for the first month, but maybe it's because they assume everyone is going to pay on a per-month basis? Or is it really for the first month?

If someone knows please clarify... I am launching soon and want to create a promo for the first 30 clients... free the first year.