Fieldsets are a group of fields treated as a single field, e.g., a fieldset named "address" with the individual fields street, zip, and city. Relations allow us to link Content Items together and provide context about the relationship, e.g., linking two images to a document and declaring that one is the hero shot and the other is the logo, or connecting all images for a 360° view of a product. You use the corresponding schema (which groups the fields) in fieldsets and relationship fields. Examples are: 

• Fieldset "address" (fields: street, zip, city)
• Relationship "Referenced content" (fields: title, media type)

The main benefit is that users only add one metadata field (type fieldset or relationship), and they get a set of fields for strictly structured data input. The fields of this schema are linked in the fields of type relationship or fieldset. Depending on the field settings, users (or external applications) can add one or multiple instances to the item.