There are variables that may or may not affect your process but are not within your control. There may even be variables that your supplier is unaware of and certainly may be ones they never measure, even if they are aware of them. If they do measure them, they may not be prepared to share the information. There can be a concern that the shared information could be used to reject product or, if the film had been coated, result in a claim. I would argue that if this variable results in a failed product, then measures should be put in place to get control of the variable and not just for it to be monitored. This is often easier said than done as the cost to produce the benefit may not be justified.
It is known that within the stenter used to stretch polyester film there is an amount of white powder from evaporated monomer from the film surface that condenses on the stenter chamber surfaces. It takes a lot of energy to heat the stenter, and so the air is recirculated and filtered. It is known that using single-pass air is cleaner and results in less white power contaminating the film surface. The cost of heating single-pass air is unacceptable to customers, as too is using finer filters, that would require more downtime to clean or replace them, and so everyone accepts that polymer film will have particulates on the surface of which many will be from the film-making process.
Particles on the film surface is another variable that is usually ignored. It is a source of pinholes in coatings and a major limitation in obtaining better barrier coatings, But it is hard to quantify and requires an additional process, and so additional cost, to try to control or reduce the impact of this variable. All film manufacturers could clean their film before they are wound up. Similarly, the slitters could all have a cleaning process included before the film is rewound. Similarly, wherever the film is wound is an opportunity for the film surface to be examined and a map of surface contamination to be produced. However, the cost of implementing this would increase the cost of the film, and so each of the suppliers is reluctant to do this as it would make their product less competitive.