what is a bucket type in composite aggregation?

I think this question, is in continuation to your previous question, so considered the same use case

You need to use Bucket sort aggregation that is a parent pipeline aggregation which sorts the buckets of its parent multi-bucket aggregation. And please refer to this documentation on composite aggregation to know more about this.

Adding a working example with index data, mapping, search query, and search result

Index Mapping:

{
  "mappings":{
    "properties":{
      "user":{
        "type":"keyword"
      },
      "date":{
        "type":"date"
      }
    }
  }
}

Index Data:

{
  "date": "2015-01-01",
  "user": "user1"
}
{
  "date": "2014-01-01",
  "user": "user2"
}
{
  "date": "2015-01-11",
  "user": "user3"
}

Search Query:

The size parameter can be set to define how many composite buckets should be returned. Each composite bucket is considered as a single bucket, so setting a size of 10 will return the first 10 composite buckets created from the values source. The response contains the values for each composite bucket in an array containing the values extracted from each value source. Defaults to 10.

{
  "size": 0,
  "aggs": {
    "my_buckets": {
      "composite": {
       "size": 3,               <-- note this
        "sources": [
          {
            "product": {
              "terms": {
                "field": "user"
              }
            }
          }
        ]
      },
      "aggs": {
        "mySort": {
          "bucket_sort": {
            "sort": [
              {
                "sort_user": {
                  "order": "desc"
                }
              }
            ]
          }
        },
        "sort_user": {
          "min": {
            "field": "date"
          }
        }
      }
    }
  }
}

Search Result:

"aggregations": {
    "my_buckets": {
      "after_key": {
        "product": "user3"
      },
      "buckets": [
        {
          "key": {
            "product": "user3"
          },
          "doc_count": 1,
          "sort_user": {
            "value": 1.4209344E12,
            "value_as_string": "2015-01-11T00:00:00.000Z"
          }
        },
        {
          "key": {
            "product": "user1"
          },
          "doc_count": 1,
          "sort_user": {
            "value": 1.4200704E12,
            "value_as_string": "2015-01-01T00:00:00.000Z"
          }
        },
        {
          "key": {
            "product": "user2"
          },
          "doc_count": 1,
          "sort_user": {
            "value": 1.3885344E12,
            "value_as_string": "2014-01-01T00:00:00.000Z"
          }
        }
      ]
    }