Fields are usually optional, something that was pre-configured by your website builder when building the site. For example, for an Event content type in GovCMS the fields have been setup as logical and necessary fields for events.
A property is set at a deeper level (sometimes referred to as "baked in"). For example, all content has a date it was created on, and this date property plus any associated functionality is fixed. It is part of GovCMS and the underlying Drupal CMS.