You've got the right answer User1971. In the case of LABTRANS updates, there are fields in the corresponding WORKORDER that get updated, and therefore WORKORDER launch points are triggered. When you update the CLASSUSEWITH there is no corresponding update in the CLASSSTRUCTURE object, hence the behaviour you are experiencing.
Could you have that as a non-persistent field instead? Then you would be able to simply calculate its value upon field initialization instead of worrying about keeping it up-to-date. If that's not an option and you do go ahead with a CLASSUSEWITH launch point just make sure you get the CLASSSTRUCTURE object via the right relationship (most likely the owner of CLASSUSEWITH in your case) in order to avoid "record has been updated by another user" errors.
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GABRIEL DIAS CESARIO
Managing Consultant
IBM
Melbourne VIC
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