The ¾" height extension sold here fits Thermo Panel 3e dog doors for sliding doors and slides into the top of the door and adds ¾" to the total height.
The height extension plugs into the top of the pet door and is secure without any additional fasteners, but it can be screwed in if desired. In most cases, this part is not necessary, but every once in a while we will come across a door that is just a bit too tall for a standard size. Example: the regular size Thermo Panel adjusts between 77¼ and 80¼ inches. The proper way to measure your sliding glass door height for a pet door is to go inside the track from the very top to the very bottom. If your door measured 80½" inches tall, the standard Thermo Panel would be just ¼" too short. By adding the height extension, the effective adjustment range becomes 78 to 81 inches, so it would work with the 80½" inch door without any trouble.
If your door is just a bit taller than the short version of the Thermo Panel or Quick Panel which adjusts from 74¾" and 77¾" inches, don't bother with the height extension. Just get the next size up, the 77¼ and 80¼ adjustment range.
If you would prefer to use a door without a height extension, the Thermo Panel and Quick Panels can be custom ordered to fit special heights. When special ordered these doors are not refundable, and it takes about 5 weeks to get them, but it is an option if a height extension isn't acceptable.