Your tier is based on your spend from the previous year and resets every October 1st.
For example, if between October 1st 2024 through to September 30th 2025 you had 500 Nation Credits, you will be in the Sexy tier for the year. Your Nation Credits will reset to 0 though and if you don't earn enough Nation credits to remain Sexy all year, the following year you will become Bold.
Please note that any returns will deduct from your Nation Credits.