It is a huge topic with lots of special cases and knowledge of the specifics of your system that simply cannot be generalized.
At a high level:
- command when you want the Rule to trigger when the Item is specifically commanded
- update when you want the Rule to trigger anytime the Item is “touched” whether or not the Item actually changed
- changed when you want to Rule to trigger when the Item changes state
Seems simple enough but what about side effects?
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command will only trigger if the Item is specifically commanded (from the sitemap, a binding, or another Rule) and if that Item is linked to a Channel or binding that command will almost certainly go out to the device as well. Is that acceptable in your case? I can’t say, only you know your system.
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update will trigger anytime the Item is updated or commanded but this means the Rule will trigger when the Item is populated using restoreOnStartup as well as from updates caused by bindings, sitemaps, and rules. Does this matter? It depends on the specifics on this Item and how it is used and populated.
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changed will trigger anytime the Item’s state changes but this means the Rule will trigger when the Item goes from NULL to a restoreOnStartup state on startup. Does this cause problems? I can’t say, it depends on how the Item is used.
So as you can see, given a very specific scenario, I can easily tell you what Rule trigger would be most appropriate, but I can’t give simple advice that is generally applicable because it depends on the specific details about how the Items in question are defined, how they are updated and commanded, and what the Rule is supposed to do.
This sentence as written does not make sense. Care to elaborate?