CouchDB附件中使用的“revpos”值是什么?

下面是一个例子:

   "_attachments": {
       "002-imgsq.jpg": {
           "stub": true,
           "content_type": "image/jpeg",
           "length": 80074,
           "revpos": 3
       },
       "016-imgsq.jpg": {
           "stub": true,
           "content_type": "image/jpeg",
           "length": 100095,
           "revpos": 4
       },
       "MASTER_img-square.jpg": {
           "stub": true,
           "content_type": "image/jpeg",
           "length": 70611,
           "revpos": 2
       }
   }