Why do i have traffic balance when im in subscription data plan?

i set it to subscription data plan but i have traffic balance and it keeps decreasing.
as fas as what i understand is subscription should charge me 1 time per item i equiped for duration of 1 month.
what will happen if i run out of this traffic balance before 1 month?

Please check FAQ - Credits link why it’s decreasing. However if your data plan is Subscription, it doesn’t last for long time.

However regarding to “Traffic Balance” message issue, please ignore at this moment. I’ll make a change to remove this section in the next release.

nevermind, i just checked the balance again and it has not been decreasing anymore
i was worried because the balance decreased by 0.3GiB by the first night, i thought it may not last me 30 days if that continues.
maybe it was just adjusting since that was the first day i bought it, thanks for your reply though.