Suppose you want the HMI react to a particular status of your machine. The PLC let’s the HMI know about this status using a special variable, e.g. MachineState which is uses Enumerations (Data Variable Property). Additionally you also want to display a Dialog for this particular which informs operators about this state state that provides for example the status code that your PLC.

Instead, you would prefer to send a unique key for this particular translation. The HMI would automatically translate this key into the language currently selected by the user.

That's precisely the purpose of the TranslationsKey data type. It can be used to let HELIO know that a particular String variable should be translated automatically.

Messages are user-visible strings, often with variable elements like names, numbers and dates. Message strings are typically translated into the different languages of a UI, and translators move around the variable elements according to the grammar of the target language.