Depending on the plan your organization has, you will have a specific limit of users and petitions.
It is considered a user any person with an account in your organization, regardless of the permit they have. So, everytime you invite someone to your organization will consume one credit.
In this article, you will learn how to invite new users to your organization.
A petition is the tool you use in Parallel to complete a piece of information, either by sending it to a third party or by completing it yourself to generate a document.
Each time you select a template and select the Create petition button, you create a specific process using such templete. That is what we call petition.
However, a credit will not be consumed until that petition is submitted or begins to be completed.
You want to learn more about petitions, in this article.