The threat spectrum is part of threat definition and presents the range of potential threats in an organized way that can be easily evaluated.  Information about potential adversaries is filled into a chart, such as the one pictured below, that designers use to compare the motivations, potential targets, and capabilities of adversaries.  

The information in the threat spectrum is used by designers to determine the likelihood a facility will be attacked and by whom, as well as to identify the highest consequence target of interest.  Anticipating the likelihood of an adversary attacking is problematic, so designers choose to be conservative by assuming all adversaries will attack the facility.  Designers then construct a security system to counter the highest consequence of an attack on the facility.