Fielddata usage problem with _type field

Elastic Stack Elasticsearch
1

Hi everyone,
I'm not fully understand what does this means when I access

The API return

    "xyzreport_20161025" : {
        "primaries" : {
            "fielddata" : {
                "memory_size_in_bytes" : 15704336,
                "evictions" : 0,
                "fields" : {
                    "_type" : {
                        "memory_size_in_bytes" : 15704336
                    }
                }
            }
        },
        "total" : {
            "fielddata" : {
                "memory_size_in_bytes" : 15704336,
                "evictions" : 0,
                "fields" : {
                    "_type" : {
                        "memory_size_in_bytes" : 15704336
                    }
                }
            }
        }
    },

What is actually _type field means? I'm sure that my mapping schema does not have this field, is this mean ES internal field?

Why and when ES will use this _type for fielddata?
Please share your idea and experience.
Many thanks