The eye mark is there to tell the machine on the pack line where to cut the film. The packs are supplied on a continuous reel. The reel is fed into the packing machine which cuts off individual packs, folds them, seals the join (creating a tube), seals one end, fills them with product and then seals the other end. The eye mark doesn't have to be at the point where the cut occurs as the machinery can be set to cut at an offset from the eye mark. The eye mark has no bearing on the crimps or seals, it is strictly for cutting. As mentioned above, the position, size and colour of the eye mark is defined by the factory / packer and all of this info should be included on the cutter guide and print spec. Generally, the eye mark needs to be high contrast (dark on a light background or vice versa) and should sit in a clear 'track' that runs in the direction of the roll as it is fed into the packing machinery. This means that no variation in colour, logos, text etc can be in the eye mark track. Some printers will add the eye mark themselves before making plates and may even move elements and adapt the design for best results, but some will insist on the eye mark being correct and included in the supplied print ready artwork. More often than not, the position will be chosen so that it is hidden or covered on the final assembled pack, usually underneath the seal on the back of the pack.