What's the LabelId reffer to?Is Label File id?

Hi， I have some questions about add shipping label option . **1. When add Label to return ,The Api said should set RMA；but It will step to next progress ,and can't upload label. question:Do need set RMA before add Label? 2. When add Label,The LabelId is requied, What's the labelId reffer to?** 3. one error : follows are api's content ;It's wrong url. > Send the shipping label to the buyer. > Call POST /return/{returnId}/file/upload to send the shipping label to the buyer.