Android CONTENT TYPE - Is vnd.android.cursor.dir some standard constant defined by android?

Documentation: https://developer.android.com/guide/topics/providers/content-provider-basics#MIMETypeReference

The MIME types returned by ContentProvider.getType have two distinct parts:

type/subType

The type portion indicates the well known type that is returned for a given URI by the ContentProvider, as the query methods can only return Cursors the type should always be:

• vnd.android.cursor.dir for when you expect the Cursor to contain 0 through infinity items

or

• vnd.android.cursor.item for when you expect the Cursor to contain 1 item

The subType portion can be either a well known subtype or something unique to your application.

So when using a ContentProvider you can customize the second subType portion of the MIME type, but not the first portion. e.g a valid MIME type for your apps ContentProvider could be:

vnd.android.cursor.dir/vnd.myexample.whatever

The MIME type returned from a ContentProvider can be used by an Intent to determine which activity to launch to handle the data retrieved from a given URI.

Where did this come from?, or can I change it like vn.com.android.myexample.dir/

No, because "vnd" stands for vendor in MIME Registration trees, android in this case.