Malicious Traffic (Cannot block) Protocol

Hi all ;

I'm quite new in Websnese Administration. What is the "Malicious Traffic (Cannot block) protocol" in Protocols filters? Why we can't block this? I've found a KnowledgeBase but there is not much information over there. Can anyone explain me with more details?

http://www.websense.com/content/support/library/web/v76/triton_web_help/security_protocols.aspx

In here, it says "no other filtering action can be applied". But we can click block? I'm a little bit confused :/

Thanks in advance.