At the moment to make an attribute conditionally mandatory I am using Business Rules to generate Error Messages but this only helps the user when they come to save the object.
It would be useful if it were possible to give a visual indicator to the user that can be switched on when the condition comes into play and off when it is no longer applicable.
Implemented as a presentation rule. Presentation rules are not dynamic, so for first creation does not always work as intended
See also Improve Validation #p45006