Custom application forms can contain both "data fields" and "user fields."
Data fields affect the person and his attributes in VSys: name, address, certifications, etc. For example, if you place a person's address in a custom application form layout, then when you change that person's address on the custom application form, it will update the address on all screens throughout VSys.
User fields do not use or affect data outside of the custom application form itself. User fields can be added to store data similar to those in custom fields, to hold comments, or just for data that you don't otherwise have a home for. Data stored in user fields is local to the custom application, and is not editable, searchable, or reportable elsewhere.