Common fields are contact fields (e.g. first name, last name, email address) that apply to all kinds of records (e.g. contact records, membership applications, event registrations, donations). Other fields apply only to membership applications, event registrations, or donations.
Each record is a combination of common fields and fields specific to that type of record. For example, a membership record will consist of membership fields and common fields, while a donation record will consist of donation fields and common fields.
The forms that appear to the user – and are used to create records – also combine both types of fields. A membership form, for example, will display both common fields and membership fields.
To customize a form, you can add or remove fields, either common fields or fields specific to the form.
The values entered in common fields on membership application forms will update the corresponding fields in the applicant's contact record (if one already exists), but the values entered in common fields on event registration forms are stored separately from the contact record within event registration records. Similarly, the values entered in common fields on donation forms are stored separately within donation records.