US bank shield visa card extendpay: how does it work?

I got approved for the card today and was reading about its extendpay benefit that kicks in after the initial promo APR period ends. Their website says

Account must be in good standing (not past due or overlimit) and meet eligibility requirements to receive the 3 month $0 fee U.S. Bank ExtendPay® Plan offer each calendar year. This offer will be available within 2 months after the introductory purchase APR offer expires. Eligibility is determined monthly and this offer can only be used once per calendar year. You may designate up to 50% of your credit card line ($100 minimum) in eligible credit card purchases.

What exactly does "this offer can only be used used once per calendar year" mean? Does it mean that I can only use it for one purchase? Or can I have multiple purchases within some time? Any ideas?

Thanks!