A custom field is field specific to your organisation that you may require in addition to the "core" fields.
You can add your own Custom Fields via Organisation Settings > Custom Fields beyond the standard fields TidyHQ provides.
Custom Fields could be a Registration Number, First Aid Certificate Number, School Attended etc.
Each Custom Field has a Title, a Type and a 'Visible to' option which will determine who can have access to the field or if it is 'read-only'.
You may also have a variety of options for your custom fields limiting the chance of errors when inputting data. Options are:
Custom Fields are quite versatile and can be used anywhere your members are interacting with your account be it Memberships or Event Tickets.
If you want to use Custom Fields to collect information from Contacts, then the "Visible to: Public Profiles" must be checked. If you want them to be able to see the file but not edit it - like Membership Numbers then you can provide them read-only access through the "Visible to: Public Profiles - Read Only" option.
The 'Visible to: Public Profile' option lets you decide if you want the custom field to be displayed on the Contact's Profile. If you choose to display the custom field on the profile, the Contact will have the ability to update it at anytime. This will automatically update the information on your side too.