What is the meaning of “bounded” and “plain” about TopicDataType ？

[wddpfx]

`

/**
 * Checks if the type is bounded.
 */
RTPS_DllAPI virtual inline bool is_bounded() const
{
    return false;
}

/**
 * Checks if the type is plain.
 */
RTPS_DllAPI virtual inline bool is_plain() const
{
    return false;
}

`

give some examples?