A framed opening in a door usually is used for admitting air and light.

Windows- Cuts are made to how you would perfer your lighting to enter or to the style of viewing you would be more interested in.

Louvers- These can help with air circulating though a warehouse, greenhouse, restroom or even a janitors closet.

Door Heights- By custom cutting a door to meet your height measurment, and to be able to fit into an existing opening height.