post /api/projectserver/v2/projects/{projectId}/changeassignment

{
           "Comment" : "Your relevant comment.",
           "Files" : [
               {
                   "LanguageFileId" : "1184ef23-50e4-4f88-b983-172c34a68ab4",
                   "DueDate" : null,
                   "PhaseId" : 1361,
                   "AssignedUsers" : ["firstUsername", "secondUsername"]
               },
               {
                   "LanguageFileId" : "3aa9239e-700f-4679-8a30-0ac7988f8168",
                   "DueDate" : "2015/12/20  10:45:40",
                   "PhaseId" : 1362,
                   "AssignedUsers" : ["secondUsername", "thirdUsername"]
               }
           ]
        }

1、what is LanguageFileId  and how to get it?

2、what is PhaseId   when and how it changes? how to get it?

3、AssignedUsers use username,not userid,comfirm?