Your environment might have versions of Identity Manager earlier than version 3.5, or you might have a mixture of different versions of Identity Manager.
Before Designer 2.1, if you configured and wrote policies for an earlier version of Identity Manager in your environment, you might have encountered the following issues:
You could easily build a solution that would not deploy.
You did not know which features worked in one environment versus another environment.
To solve these issues, Designer 2.1 tracks versions of the following objects:
Identity Manager engines
Identity Vaults (trees)
As you use Designer 2.1, you see only the UI of features that apply to the version that you are working on. Project Checker and Deploy ensure that what you have configured is supported in the target environment.