When you add this post-function to a transition and trigger the transition, the add-on sets the selected issue security level on the current issue, if and only if the current user belongs to the selected project role.
This function can be used on the Create transition to set a different issue security level depending on whether the issue is being created by an internal user or by an external user (e.g. a customer).
Conditional execution
To execute this post-function based on the result of a groovy expression see Conditional execution using Groovy expression.