Answer
For a change affecting only yourself, make it directly in your own Personal Profile; for a change that should affect multiple users, make it in a Base Profile instead, since each user is connected to their own Personal Profile by default but can additionally be linked to a shared Base Profile layering common behavior across many users. Under Options > User Profile, choose Edit Base Profiles for a multi-user change and select the target base profile, navigate to the relevant screen, RMB > Properties, and use the Mandatory Fields tab there to select which specific fields should become mandatory. Save the profile afterward, confirming you're saving to the correct profile name — once users log back in, the updated mandatory-field behavior takes effect for them.