For long term storage, films must be stored horizontally. This evens the weight of the film over the whole surface area of the side of the film pack. The horizontal orientation also ensures that the tension in the film is even throughout the film pack.
Access materials are often stored vertically, usually because of space restrictions and ease of access requirements of the materials. However, this is not ideal from a preservation point of view.
If a film is stored vertically there is only a small area of the film pack supporting the weight of the entire film. This often results in accelerated dye fade or a higher degree of decomposition occurring in such an area.