It depends on whether you've acitivated plan auto-renewal feature or traffic auto-recharge feature in the subscription management.
If there is any available balance in your account and you've activated plan auto-renewal feature, when the plan is about to expire, the system will attempt to deduct the fee and automatically renew your package for you. Under that circumstance, when the last plan comes to an end, the new plan will work. The traffic and period of the plan will be refreshed.
If there is any available balance in your account and you've activated traffic auto-recharge feature, when your traffic for your plan is about to run out, the system will attempt to deduct the fee and automatically add traffic for you. Under that circumstance, please pay attention that there is only the change of traffic upper limit, but the period of the plan will not be extended.
For example, I ordered a 1GB/month plan for Residential Proxies on Oct 12th, the period of the plan is from Oct 12th to Nov 11th. Then, I enter the subscription management page and set traffic auto-recharge feature for 1GB each time. When it's Oct 25th, I've already used 900MB for this plan and it's already reached 90% use of the whole plan, the system will try to deduct the fee from the balance. If it's been done successfully, your plan will be supplemented with an additional 1GB of data, but the duration of the package will not be extended, it will come to an end on Nov 11th.