You can place multiple orders within a project. If you currently have a pending order, you can place translation orders for new phrases without affect your pending orders.
Untranslated phrases in your pending orders will be considered as "already translated" when placing new orders.
For example, in your project Veggie Run, you have the following English (en) phrases:
You place a translation order for Strings A, B, and C for French (fr). Strings A, B, and C are currently pending and have not yet been translated by our French translator.
You upload the following strings:
Project Veggie Run now has a total of 5 phrases. You want to translate strings D and E, but strings A, B, and C are still pending and not yet translated.
You can place an French order for strings D and E without affecting the pending translation orders for strings A, B, and C (which will be considered as "already translated").