For example, I have, in my termbase, a source term sourceA, and a target term targetB which is set to Status (System): Preferred, plus another target term targetC which is set to Status (System): Forbidden. 

The purpose of this is to emphasize that the source term sourceA should only be translated to targetB, and should definited not be translated to targetC. 

When the option "Terms which may have been set as forbidden" is enabled, and I select Status (System) for Picklist Fields, then check Status (System): Forbidden, I want Trados to first recognize source term sourceA in the source segment, then, in the target segment, if targetC is identified, report it ERROR. 

But now, whenever targetC is identified, Trados reports ERROR regardless of whether source term sourceA exists in the source segment or not.