Permissions for a role in RTC

In RTC we created a rol called "Customer." For this rol, it is necessary that only has permissions to modify a field in a specific work item (Requirements).
The thing is, even if we change the permissions settings, the rol can modify all fields in the work item. Why does this happen?
The thing is, even if we change the permissions settings, the rol can modify all fields in the work item. Why does this happen?
One answer

In RTC we created a rol called "Customer." For this rol, it is necessary that only has permissions to modify a field in a specific work item (Requirements).
The thing is, even if we change the permissions settings, the rol can modify all fields in the work item. Why does this happen?
What can the Everyone role perform ? RTC seems to be an inclusive permission model rather than exclusive. That is, for a given activity, the permissions look to find
some role that permits the activity, not prevents.