CoRE is a very powerful community – created rules engine for SmartThings. It allows you to set up stacked conditionals like "If A then B but only while C and not if D."
Setting it up can be quite complex, but there are many community members who will be glad to help you.
There are two threads that you will need.
The first is the peer assistance thread. Feel free to ask any questions about what CoRE can do or How to set up a specific rule in CoRE in that thread and someone will help you. (CoRE calls an individual rule a "piston.") It's also a good place to look for samples of Pistons, since many people will post screenshots there.
For example, here is a piston that one member created to notify him if the dog had already been fed twice that day and he picked up the dog bowl for a third time. He needed this because he has a large family and people didn't always remember whose turn it was to feed the dog.
The second is the author's thread, where you will get the actual code.
There's also extensive documentation on CoRE in the community-created SmartThings wiki.
The author of CoRE is working on a new version, WebCoRE, which moves the data entry to a web interface. This allows for flexibility and an architecture that requires more memory than is available through the mobile app. You will see a number of references to this in the forums, but be aware that it is still in early beta development and as of this writing the original core still has more features, although the UI is easier to use in webcore
WebCoRE also has its own wiki with a lot of information:
Please do not ask questions about how to use CoRE in this thread, as you will get much faster and better answers by asking in the peer assistance thread where the CoRE experts hang out. This FAQ is just here to briefly answer the question of what CoRE is, not to answer questions about how to use it.
Note: you may see references in the forum to an older community – created rules engine, "rule machine." Rule Machine was discontinued in the spring of 2016. When no official rules engine took its place, other community members, particularly @ady624 , developed CoRE.
CoRE has since moved considerably beyond where rule machine left off in both power and complexity. So anything that could previously have been done in rule machine can now be done in CoRE, although you may need expert assistance to design a piston to do so.