The term deliberate release refers to any intentional spread of a biological agent, such as might occur in a biowarfare or bioterrorist incident. A release may be overt or covert.
An overt release is clearly identifiable - there may have been prior warning, or the release may be apparent, either due to the use of an explosive device, or because a suspicious substance is obviously visible. Alternatively a release may be covert, with a concealed release of an agent. In these circumstances, the release will not become apparent until the first cases of disease arise.