How Do I Add User Data Fields To An Invoice Or Notification?

I have been looking for a couple hours and cannot find the answer. When a customer checks out they are asked to submit user data fields, such as "How did you hear about us?" How do I include this information in a notification to admin, or in the Invoice that is generated?

I know how to access the area for editing notifications and the invoice, but I do not know the proprietary CS-Cart code to add the user data fields.

Any help would be much appreciated!

It depends on the section where the field is located. For example, for the Contact information section use u.c_fields.45.value and

where 45 - profile field ID (can be checked in the admin panel)