This is the Android platform's base MIME type for a content: URI containing a Cursor of zero or more items. Applications should use this as the base type along with their own sub-type of their content: URIs that represent a directory of items. For example, hypothetical IMAP email client may have a URI content://com.company.provider.imap/inbox for all of the messages in its inbox, whose MIME type would be reported as CURSOR_DIR_BASE_TYPE + "/vnd.company.imap-msg"
Note how the base MIME type varies between this and ContentResolver.CursorItemBaseType depending on whether there is one single item or multiple items in the data set, while the sub-type remains the same because in either case the data structure contained in the cursor is the same.