In the Version Control Settings dialog, the This model is private option affects the availability of the Get Latest / Get All Latest menu options. These options cannot be used with shared models. This is to prevent the possibility of overwriting any new modifications to a package with out-of-date versions of the package that are retrieved from version control.

Consider the following scenario, with two users both working on a shared model at the same time.

If user1 checks-out and modifies a package within Enterprise Architect, user2, who also has this shared model open in Enterprise Architect, can see the changes to the model immediately.
 
If, at this point, user2 performs a Get Latest or Get All Latest on the package, they retrieve from Version Control a package that is now out of date. (User1 has it checked-out and has made changes that are yet to be checked-in.) The out-of-date package would be imported into the model loaded on user2's machine, immediately updating the model database (either the shared .EAP file or the DB repository) and overwriting the modifications that user1 has just made.

Similarly, if a user checked-out and modified a package and then performed a Get Latest on that package, the modifications would be overwritten by the version of the package retrieved from version control.

The Get Latest option is consequently disabled for packages that you have checked out.