The Threat Scope is the name for two different kinds of attachment in Titanfall 2. They both take the form of optics providing an AR overlay on their scopes, highlighting enemy troops in red.
However, the main difference is in size; assault rifles, SMGs, Light Machine Guns and shotguns all get one that is more comparable to a Holographic Sight, while Sniper and Marksman weapons get a full scope. Every single primary weapon, with the exception of the Grenadier class, has compatibility with one variant of threat scope or the other.
Due to the power offered by highlighting enemies, Threat Scopes also take up the second mod slot for their weapon, meaning a Pilot must choose between having two mods and another sight, or a single mod and the Threat Scope.