What is the _COUNT column for?

Android specifies the super-handy _id column in the BaseColumns interface, usage of which is very well explained in this question, but what is _count for? Count of rows in a directory, what directory?

public interface BaseColumns
{
    /**
     * The unique ID for a row.
     * <P>Type: INTEGER (long)</P>
     */
    public static final String _ID = "_id";

    /**
     * The count of rows in a directory.
     * <P>Type: INTEGER</P>
     */
    public static final String _COUNT = "_count";
}