We are using ServiceDesk Plus Standard Edition for our Admin and HR Team. There is a dedicated coordinator who routes/assigns the ticket to the concerned person. Once the ticket is assigned to the concerned individual he or she should not able to modify the category,sub-category,item and priority. 

I am not able to find the correct logic on when and how  to trigger this rule using Form & Field rules. Your assistance on this would be highly appreciated. 

Regards,

Shri