Often, all types in an index share similar fields and settings. It can be more convenient to specify these common settings in the _default_ mapping, instead of having to repeat yourself every time you create a new type. The _default_ mapping acts as a template for new types. All types created after the _default_ mapping will include all of these default settings, unless explicitly overridden in the type mapping itself.

For instance, we can disable the _all field for all types, using the _default_ mapping, but enable it just for the blog type, as follows:

PUT /my_index
{
    "mappings": {
        "_default_": {
            "_all": { "enabled":  false }
        },
        "blog": {
            "_all": { "enabled":  true  }
        }
    }
}

The _default_ mapping can also be a good place to specify index-wide dynamic templates.