EclipseLink/UserGuide/JPA/Basic JPA Development/Configuration/JPA/Defaults
Each annotation has a default value (consult the JPA specification for defaults). A persistence engine defines defaults that apply to the majority of applications. You only need to supply values when you want to override the default value. Therefore, having to supply a configuration value is not a requirement, but the exception to the rule. This is known as configuration by exception.
Note: You should be familiar with the defaults to be able to change the behavior when necessary.