Each document indexed is associated with an id and a type. The type, when indexing, is automatically indexed into a _type field. By default, the _type field is indexed (but not analyzed) and not stored. This means that the _type field can be queried.
The _type field can be stored as well, for example:
{
"tweet" : {
"_type" : {"store" : "yes"}
}
}
The _type field can also not be indexed, and all the APIs will still work except for specific queries (term queries / filters) or faceting done on the _type field.
{
"tweet" : {
"_type" : {"index" : "no"}
}
}