When are assignment rules evaluated?

Question

Does anyone know when assignment rules are evaluated? 
 We have an expression rule that returns a Group based on a set of parameters. This expression rule can be used for swimlane assignments, attended task assignments and for process model alerts. In order to make most efficient use of this expression rule (and understand the trade-offs between various options) we are keen to know when/if the rule would be evaluated for each of these scenarios. 
 E.g. 
 
 If the expression is used in a swimlane assignment will it be evaluated every time a new instance of that process model is initiated or only if an attended task in that swimlane is assigned based on that swimlane assignment (and if so would it be evaluated twice if there were two attended tasks in that swimlane)? 
 If the expression is used directly in the assignment of an attended task will it only be evaluated if that attended task is triggered or is it always evaluated when an instance of that process model is initiated? 
 If the expression is used in the process model alerts tab will it be evaluated every time a new instance of that process model is initiated or only if an alert is raised for that instance?

Chris · Answer

Agree with the others that the assignment expression is evaluated at task start time , regardless of if it is in the node assignment tab or in the swim lane assignment. So with either configuration, if the group is updated and the task restarts later, it will utilize the new assignee set. 
 I assume the same for the process model alerts recipients, although we have never tested - all our alerts come to system admins only. 
 You could test this out by creating a simple model with a script task with a bad expression such as "=if(true,false)" (incorrect number of parameters). Start the process for debugging, not the recipients, update the group, manually start the bad node again in the active/test process to see if the recipients changed.