def is_linked_release(data):
    """
    Returns whether the data is a linked release.

    A linked release has required ``url`` and ``date`` fields and an optional ``tag`` field. An embedded release has
    required ``date`` and ``tag`` fields (among others), and it can have a ``url`` field as an additional field.

    To distinguish a linked release from an embedded release, we test for the presence of the required ``url`` field
    and test whether the number of fields is fewer than three.
    """
    return 'url' in data and len(data) <= 3