This page explains about its features and abilities in detail.
Click on the Create issue.
Create an issue without assigning a user.
It will show the error message defined when adding the expression and error message in the validator when validation fails.
Now assign the issue to any user and create it. It will not show the error message.
Next page:
https://confluence.prod.mgtechsoft.com/wiki/spaces/EFJ/pages/edit-v2/3057680385