The article is clear on how the price index of sumcoin works and what you must do in order for the trade to get executed successfully.
1. Your buy or sell order price has to be within +1% or -1% of the index price at that specific time.
2. Existing orders will also get automatically cancelled should the increase or decrease by more that 1% from your order price.
If you are still confused on how Sumcoin index is traded, you can also contact their support -
support@sumcoinindex.com