Trijicon's SNIPE-IR series of thermal Electro-Optics is a dedicated clip-on device that is designed to be used in conjunction with a mounted and zeroed day optic to provide thermal target detection capabilities.
While other Trijicon Electro-Optics like the REAP-IR have the ability to serve as a clip-on if needed, only the SNIPE-IR is designed from the ground up to serve in this role.