Normally when a field has an "optional" attribute, the field will be visible only when the option appears on the URL. But when the option starts with '!', then the field will become invisible if the option (without the !) appears in the URL, and will be visible only when the option doesn't exist. For example:

When the url is http://localhost:8003/
SSN will be visible, but NickName will be invisible.
When the URL is http://localhost:8003/?internal
SSN and NickName will be both visible
For http://localhost:8003/?internal&_UK_
only NickName will be visible, and SSN will be hidden.
http://localhost:8003/?_UK_
will make both invisible