The overwhelming majority of window film is installed on the inside of a window, although it can be installed on the outside. There are pros and cons to each type and it depends on the product and situation.

Most installations are interior-applied for a few main reasons. First, interior-applied films last longer because they are not subject to the elements (rain, snow, wind, airborne contaminants, etc). Second, it is easier to install an interior-applied film working from the floor and a ladder than it is outside on a lift or swing scaffold. Third, interior-applied films can usually be installed year-round while exterior-applied film installation depends on the weather. (It can’t be too cold or windy.) Finally, given the easier nature of interior-applied films, final turnkey installation costs are substantially lower than for exterior-applied films.

Exterior-applied films are suitable when access to the inside is restricted, impossible, or difficult. Also, they are generally safe for triple pane glazing, which can be subject to thermal stress with an interior-applied film. Finally, exterior-applied films offer better solar heat gain reduction because the film is rejecting energy before the energy passes through the window.
Most decorative films such as 3M Glass Finishes (Fasara, Dusted / Frosted Crystal, Dichroic) are installed on the inside.