CORE Micro is designed to be modified. There are two basic approaches: Skinning and Hacking.
When Skinning CORE Micro, you are essentially taking the CORE system and laying a setting on top of it -- complete with worldbuilding stuff, character stuff, money/wealth stuff, perhaps monsters, factions, etc. You may change the names of Stats or other things, but you're not really writing any new rules that affect the CORE mechanics. If you know your setting material well, this is extremely easy to do. You can prep a session or a whole campaign simply by "translating" characters, locations, and items to the CORE scale.
- Skins are "For Use With CORE Micro."
To run your Skin, someone would need a copy of the core rules.
When Hacking CORE Micro, you're not only changing the names of Stats or laying a world on top, you're extending the mechanical logic of the system to invent your own tables, skills, and other important subsystems, making a complete game in its own right. This is a lot more work, obviously, and might require some playtesting to make sure it all hangs together. Since you gone this far and changed so much, it makes sense to rewrite the CORE rules (or simply include whole sections) within your own work, thus creating a standalone game that could be played without having a copy of CORE Micro handy.
- Hacks are "Powered By CORE Micro."
Standalone games based on CORE, they can be run without requiring the core rules.