Besides inserting fields that belong to the post, you can add any other fields to CRED forms. Non-post fields can serve for anything that you need, which doesn’t go into the database with the post.
In the form editor, click Insert Generic Fields and the following dialog box open.
You can choose from different field types. Clicking on one will open an additional dialog, with the field’s options.
The generic fields that you add to the form are not part of the post, so CRED will not process them automatically. You are free to use generic fields for whatever your application requires. Your processing should go into PHP code and run on CRED hooks.