Your designer works on your design queue in the order they arrive: on a first-come/first-served basis. If you need them done in a different order, you can always open the ticket and change its priority (see below):

Urgent – Needs to be delivered as soon as possible (max 1 business day).
Do not mark more than 1 ticket as Urgent or it might cause confusion.
High – Tickets marked as High will come right after the Urgent ones.
You should aim to have only 2 or 3 tickets marked as High.
Medium – Next ones in line, normal order.
Low – To be done whenever nothing else is a priority.
If you need them done in a specific order, let us know in a reply to any of your design requests and your account manager will adjust the priorities for you.
If you're creating a design request via email, just write "Urgent" in the subject line before your request.
Example of email subject:
URGENT – Christmas Facebook Advert