The Customizer isn't just for standalone S3C. It includes a "Sonic 2 Merging" function. Because of how the Sonic & Knuckles lock-on technology worked (it literally mapped ROM banks), the Customizer can lock Sonic 3 Complete into a Sonic 2 ROM. This creates:
The Customizer is broken down into logical categories. Let’s walk through each one. sonic 3 complete customizer