What is SVG?

Not pixels but instructions - shapes, paths and text that a browser draws at whatever size you ask for. That makes it perfect for logos, icons and diagrams, and useless for photographs. Converting an SVG to any other format here is the moment it stops being able to scale.

What is ICNS?

Apple's icon container, the thing inside every .app bundle that makes it look like something. Like ICO it holds a set of sizes, but it goes considerably larger - Retina displays ask for 1024px icons, which Windows never did.