The answer depends on your decal type, but it changes how long your graphics last and how sharp they look from the sidewalk.
Business owners printing storefront decals for the first time almost always ask the same question: does this go on the inside or the outside of the glass? The honest answer is that it depends on the adhesive type, the material, and how much weather exposure the decal needs to survive. Back-adhesive decals are built for the outside of the window, while front-adhesive decals, and window clings are usually meant for the inside. Getting this right the first time saves you a reprint and keeps your branding looking clean for years.