A unit alias is a name used as a placeholder for an actual unit or unit class. A unit alias ensures that all unit procedures associated with this alias will run on the same unit.
The unit alias can represent a fixed unit (the unit is specified at recipe configuration) or deferred (the unit is selected when the recipe runs). The unit alias can only be defined within a procedure and used in a procedure's steps (that is, the unit procedure that the step references is associated with the unit alias).
Using unit aliases allows multiple unit procedure steps to be bound to the same unit (through the use of the same alias on each unit procedure). You can configure multiple unit aliases for each procedure with each unit alias representing one unit class (if alias resolution type is defer) or one unit (if alias resolution type is fixed).
We recommend that if any step within a recipe utilizes unit aliasing then all steps (within the procedure) that have class-based equipment assignments use unit aliasing also.
Create the needed unit aliases in the procedure. Each unit alias is procedure specific. That is, multiple procedures cannot share the same aliases. As you create steps in the procedure, associate the steps with the appropriate unit alias.
Resolution of a fixed alias occurs when the unit alias is defined. For deferred aliases, resolution to a unit occurs at the step (even when the Acquire unit prior to starting unit procedure option is selected. All of the rules pertaining to binding equipment apply to resolving unit aliases at runtime.
Unit aliases are also used when creating equipment trains. Equipment train classes are defined by associating a unit alias to a unit class. The unit alias is then referenced in the procedure and the units in the unit class that are defined in the equipment train instance, are available to the operator.