The reason why ST goes with 10% and above is because dimming below 10% with LED is not reliable and consistent with so many brand and different standards out there. Smart bulb doesn’t seem to have problem with this but standard LED is all over the place below 10%. CoRE is a community created smartapps by guru @ady624 that could virtual do anything you want.
What @Navat604 said with regard to the 10% limit. There are a number of individual devices, particular some Z wave switches, that won’t reliably dim that low.
As for core, it is a very sophisticated community – created rules engine for use with SmartThings.
If you haven’t used custom code in SmartThings before, it’s pretty straightforward. Basically you copy the author’s code and paste it into your own account. (or in some cases you can use github integration, but the author will say if that’s available.) anyway, if you read the following custom code FAQ, it will introduce you to the terms and concepts:
After that there are two important core threads. The first thread is the author’s thread. That’s where you will get the current code and you could ask any questions about the installation.
The second is the peer assistance thread. That’s where community members will help you set up a specific rule (which core calls a “piston”).
Core is very powerful, but also very complex, so don’t worry if you do have to ask for help at any point.