The basic speed of the film is set at the time of manufacture. This means that a certain amount of light will produce a certain amount of density, given a certain amount of time in a certain developer.(posted 9335 days ago)From there, things get interesting. Ansel Adams admonished in his books that the photographer should always experiment with films and papers to find out what works for the individual. This means that what you use isn't what may work for your neighbor.
Let's take an unusual but definitely illustrative case: Infrared films.
Infrared film was first developed by Kodak in the 1940's for use in spotting enemy camoflage installations. The properties of infrared light cause it to be absorbed by dark aritificial structures, but reflected back brilliantly by dark folliage. This means that plants show up white, and factories or barracks show up black.
The development times given by the IR film manufacturers give a very high contrast index. This is fine for spotting cammoflage or police photography, but it's lousy for delicate and subtle artwork. Also, the Kodak and Konica films aren't even given an ASA rating. They give a starting spot, but then you must adjust it from there.
So what to do? You have to make a series of test exposures. Just take it and shoot up a few rolls, and carefully note down the conditions, the amount of light, shutter speed, and aperture value. Maybe even do things like pre-exposure to reduce contrast.
Now, the IR film starting points from the manufacturers give extremely high-contrast images. I found that out quite quickly. So I reduced the time I was giving for HC-110 dil. B. That was OK, but still the images had too much contrast. Now I have switched from HC- 110 to Xtol. The images are much better.
Then the image needs to be printed. So what contrast goes best with which paper? That's another thing which needs to be answered.
So with relentless and scientific experimentation, a photographer will be able to figure out what works the best. An remember, it's all for the individual, which means you. Or me.
I recommend that you read the Ansel Adams series, "The Negative", and "The Print". There are also many other fine books out there. The best thing to do, though, is just experiment and keep very good notes.