Which models can this be used with?
Compatible with select sizes of Framed Floor Vent [Lite]Framed Wall Vent [Lite]Flush Wall Vent [Lite]

Can I mount this after I install my vent?
It depends on the vent. For the Framed Floor Vent [Lite] and Framed Wall Vent [Lite] you can remove the vent from the duct, and clip on at any time. For the Flush Wall Vent [Lite], the damper must be clipped on before installing the vent into the wall. 

Can I remove this once I've mounted it to my vent?
No. Once this is clipped onto the vent, the damper cannot be removed.