ISS is in low Earth orbit, and as such, its orbital progress is gradually slowed by atmospheric drag and consequently requires occasional reboosts. It also has to be occasionally maneouvered to avoid collisions with orbital debris. This all contributes to an environment aboard ISS which is not truly zero-g. Does this affect or limit the science or specialized processing which can be done when zero-g is called for? Is the difference between ISS microgravity and true zero-g small enough that it can be disregarded for the purposes of either experimentation or specialized manufacturing? Or are there processes and/or experiments for which the ISS is not useful simply because its microgravity is not a good enough approximation of zero-g? How does reboosting and other maneouvering affect the onboard microgravity environment? Do experiments/processes have to be suspended or scheduled around these events due to the accelerations they introduce? Are there long duration experiments/processes for which "normal" ISS microgravity would be adequate but cannot be done there because of the rate of interruptions by station maneouvering?
The ISS microgravity is generally considered to be of pretty poor quality, but the major cause of that is vibration from mechanical equipment and astronaut movement rather than atmospheric drag and reboosts (which are infrequent).
For many experiments it's adequate, for others it is not - alternatives include drop tower tests and drag-free satellites such as GOCE. The latter is rather clever: You put a test mass inside a cavity in a free-flying spacecraft. You measure the test mass' position relative to the cavity with lasers or some other means. As it drifts toward the edge of the cavity, you fire (extremely low-thrust) thrusters on the spacecraft to move the cavity walls away from the test mass. This ensures that the spacecraft follows the same trajectory that an ideal mass would, free from the effects of drag, solar radiation pressure and other disturbances.