Inside or Outside Mount refers to whether the product will be mounted within the window frame (inside mount) or on the wall, window trim or molding surrounding the window (outside mount).
On inside mounts, deductions are taken from the width measurement to allow the product to fit inside the window frame and provide adequate operating clearance. Minimum depth requirements vary depending on product.
On outside mounts you will want to overlap or extend the window covering beyond the window opening. You will need to determine how much overlap you want and add that amount to the window opening width and / or height measurements.