Golang _ struct {} inside a struct. Whats the purpose of it? [duplicate]

In below definition of a structure, Whats the use of _ struct{} field? Why its there?

type DeleteItemOutput struct { _ struct{} `type:"structure"` Attributes map[string]*AttributeValue `type:"map"` ConsumedCapacity *ConsumedCapacity `type:"structure"` ItemCollectionMetrics *ItemCollectionMetrics `type:"structure"` }