A phone case design template is a PSD (Photoshop Document) file used to design a phone case for a specific phone model making sure that no part of the design is cut due to the camera and speaker openings. It has a single layer made up of an outline of the phone model along with the camera and speaker openings.
Here is an example of an iPhone 6 design template. The height and width of this document are equal to the edge-to-edge height and width of the actual iPhone 6 phone case. The outline denotes the front portion of the phone case and everything outside is the edge of the phone case which has the volume rocker buttons. It is mandatory to always cover the entire rectangle with the design to avoid any unprinted patches on the phone case.