After selecting a workflow, which will be triggered by the call activity, the version of the workflow has to be specified in order for it to be executed. It is possible to select "Latest" instead of a specific version. This results in the latest version of the workflow always being triggered at runtime.
The contracts of workflows might change from version to version, meaning that number and types of input and output parameters might vary. One must ensure that the contract which was valid when designing the Call Activity must not change. If, at runtime of the workflow, there is a new work-flow with different contract, the workflow execution will fail. To ensure consistency, you should always select a specific workflow version. It might be acceptable to use "Latest" when designing it, but once your workflow is finished, and potentially used in production, you should select a dedicated version.