The Share Alike aspect requires all derivatives of a work to be licensed under the same (or a compatible) license as the original. Thus, if a person were to use parts of a BY-SA movie to create a new short film that new short film would also need to be licensed as BY-SA. The advantage of this license is that future users are not able to add new restrictions to a derivative of your work; their derivatives must be licensed the same way. This is in contrast to the Attribution-only license from CC where a derivative could be released under full copyright restrictions (All Rights Reserved).
As changes have been made to the Creative Commons licenses over the years there have been 4 versions of the BY-SA and BY-NC-SA licenses (1.0, 2.0, 2.5, and 3.0). Creative Commons has taken steps to ensure that works licensed under one particular version of the Creative Commons Attribution-ShareAlike license can be incorporated with a work from a later version. Specifically, any work that is licensed with a BY-SA or BY-NC-SA version 2.0 or higher can be used with any later version of the license. For specifics see the chart below.
|A Derivative can use the corresponding license version:|
|If the original work was:||1.0||2.0||2.5||3.0 |
|BY-SA version 1.0|
|BY-NC-SA version 1.0|
|BY-SA version 2.0|
|BY-NC-SA version 2.0|
|BY-SA version 2.5|
|BY-NC-SA version 2.5|
|BY-SA version 3.0 |
|BY-NC-SA version 3.0 |