Hey @corosc I don’t believe we can do this through Business Rules. A workaround might be to set the field as non-mandate when you select specific sub-category values and modifying the display property to hide the element using HTML under Admin>Helpdesk Rebranding>Portal Customization>Layout & Pages > Portal Pages | Tickets | New Ticket.

Having more granularity under business rules is something you’ll want to create a support ticket for so it can be raised as an Enhancement Request. It may or may not be already on their radar. Support may even have a solution to assist with the HTML part or provide a better solution.