region coding

[Spider1981]

With Blu-ray, studios have the option of region coding, but it isn't mandatory.
For instance, a new release which is coming out on Blu-ray, but has not completed it's theatrical run in another country may be region locked, but catalog titles often aren't.

While some studios, such as Warner Bros. make most, if not all of their movies region free, studios like Fox make many, including catalog titles region locked.