Unit class aliases can be used for two different purposes: by the controller when executing the phase logic; and, as a reference to a parameter collected in the Continuous Historian.
The controller-based alias is an alias used by logic executing in the controller (for example, phase logic).
The history-based alias is a reference to a parameter that is selected for history collection by the Continuous Historian. The history-based alias is only available when the ProfessionalPLUS Properties dialog has the Enable History Analysis for Batch option selected.
Once a history-based alias is configured, renaming it results in the original name being replaced with the new name in the history table. The original name will no longer be available, except if an archive was created with it. Deleting the history-based alias removes that alias from any historical data, except if an archive was created. In addition, it is not recommended to change the resolution of the alias after historical data has been collected. Instead, create a new alias and resolve it to a new data point.
Unit class aliases are assigned to the unit class (you can create more than one) and are inherited by the unit modules as unit aliases. The resolution (whether for history or phase logic purposes) is determined at the instantiation of the unit class (the unit module).
The alias used in the phase logic gets resolved (in the path field) to the actual module path being used at runtime. For example, the alias INLET can resolve to either R1_XV1/DC1/SP.CV on Reactor 1 or to R2_XV1/DC1/SP.CV on Reactor 2.
The alias used for history collection must be resolved to a parameter that is marked for history collection. To enable history collection on a parameter, right-click the module and select the History Collection option. Then add the parameter.
Unit aliases are also used in recipes as a generic name that represents a unit and is associated with unit procedure steps in a procedure.