When your traffic allowance is crossed and you do not have extra traffic available in your account, you can still use your seedbox, however your uplink will be limited to 2-5Mbit per second until
the end of your billing cycle when your traffic will be reset and your upload speed restored back to normal.
If you want, you can buy extra traffic form within your client area (in the seedbox details section). This will restore your normal
speed and any extra traffic not used until the end of your billing cycle, will be transfered to the next month too.
For your reference, your client area is located here: