As posted above, when you meter at a setting different than the ISO speed of the film, you are using instead an Exposure Index or EI.
The ISO (or older ASA and DIN) light sensitivity ("speed") standards are a measure of how a film responds to light under certain defined circumstances, which are process related. If your meter is working properly, your metering technique is a standard one, you use commercial processing and have machine prints made, there is a good chance that the best setting for your meter will be the ISO speed for the film. For most films, that ISO speed is the "box" speed.
Some films are frequently used under circumstances that are significantly different than the ISO standard circumstances. The Ilford and Kodak films with "3200" in the name are examples of that. They do have ISO ratings (800 - 1000) but they are designed to respond well to less exposure than would normally be given to film metered and exposed at the ISO standard, with a corresponding increase of development to improve rendition even if shadow detail is still less than ideal.
In addition, many people use film in circumstances where they have more control of the results than is provided by traditional commercial processing and machine prints. Ralph Lambrecht is certainly an example of that! His choice to use an EI that is 2/3 of a stop lower than ISO gives him results that take advantage of the additional control available to him. Those who use the Zone System to expose and develop film are using criteria for development that mandates a result that is 2/3 stop less.
Some photographers also prefer results with non-standard rendition of tones. Examples would include photographers who feature deep shadows and hard contrast. They achieve those results through a combination of controlling light, modifying exposure (through choice of EI and metering technique) and modifying development.
Referring to the Cinestill materials will tend to confuse the issue. They are motion picture negative film that has had the anti-halation remjet removed. The material is originally designed for ECN process, not the C41 that people are generally using to develop their still shots. All of those factors make discussions of ISO at best, problematic. There are ISO speed standards that apply to the original motion picture stock, but the removal of the remjet and the change in process essentially make that ISO speed rating nearly irrelevant.
