What's the purpose of this empty struct declaration in a GO struct [duplicate]

I was looking in the AWS S3 go code,

https://github.com/aws/aws-sdk-go/blob/master/service/s3/api.go

and noticed the return object from a put starts like this.

type PutObjectOutput struct {
    _ struct{} `type:"structure"`

What's the purpose of that first empty declaration? Is that used in reflection? So you can see if this struct is a type of "structure" or something?