The ISO setting and exposure compensation have the same effect, they change the response of the meter, but it’s helpful to separate their use so you’re less likely to get confused:
Think of the ISO you set on the camera as being locked to how you will develop the film. So in your case you may be using a 400 speed film, but using it at ISO 200. To pull it, you would then develop it as though it was an ISO 200 film (pulled one stop).
The exposure compensation is used when taking each photo, I.e. not related to the development. For instance, if you went out in the snow, you may want to add one or two stops to the exposure compensation to make the snow white instead of muddy grey on the negatives.