What is submission storage?

There is an alert in my account that I am out of submission storage but I've already deleted/purged all of my old forms and submissions. Why does it keep showing up?

Submission storage is nothing but the total number of submissions across all the forms in your account. For Free accounts, the submission storage is limited to 500 submissions only. If you have more than 500 submissions in your account, the oldest submission will be deleted when you receive a new submission.

You may upgrade your account to a paid subscription plan in order to have unlimited submission storage. 

Hope this information helps!