query Items($limit: Int!) { // The name of the class that will contain a query from the file schema.json
   customerItem(limit: $limit) { // The name of the query from the file schema.json
      ...CustomerItemInfo // GraphQL fragment that is described below
   }
}
query UserInfo {
   currentCustomer {
       ...User
   }
}
mutation AddItem($source: String!) {
    createItem(source: $source) {
        ...CustomerItemInfo
    }
}
fragment CustomerItemInfo on CustomerItem { // CustomerItem - type from the file schema.json. That means your fragment contains fields from this type object
   id
   brand
   price
   title
}
fragment User on Customer {
   email
   full_name
   id
}