Time UoM are almost Master Data for timesheet modules. Some user remove those before installing timesheet modules, making it impossible and crashing (as some field required a time UoM to be set at installation). Since we don't want to hardcode xmlids of 'Hours' and 'Days' time UoM, we rather prevent the deletion of such UoM in the system. The UoM can be desactivate to avoid having plenty of them in system in use. Task #39079