In the Delay and When functions (because of technical reasons) the unique identifier must be given (’_UID’, arbitrary string) that can occur only once in a ruleset. If you tried to use it several times, than the reader will signal error. It is recommendable to start the ’_UID’ arbitrary string with a ’_’ character, because this will make it easier to read in the debug text. You can write 0 instead of UID, then the identifier will be generated automatically. You can only give an identifier this way, if there might be a chance that the event has to be stopped early (with ClearTimer), WHICH MEANS NEVER, or if you want to make the event easily findable, from debug point of view.