Adding heat vision in front of a day optic is a fundamentally different proposition from bolting on a thermal sight that replaces it. Both put a thermal picture in front of your eye. What changes is magnification, where the weight sits, how much setup is involved, and how many rifles the thing can serve. If you're still getting your head around how any of this technology works before worrying about format, it's worth reading a plain-language primer on what a thermal scope actually does first.  Visit - https://www.eurooptic.com/blog/post/thermal-clip-on-vs-dedicated-thermal-scope