Hard Coat Include Duplex Compression

Hello, does anyone of you know what is below remarks try to say:

Hard Coat all sides 50+/- 5 microns including Duplex Compression

Material is Aluminium alloy 7075-T6

I believe HARD COAT is referred to Hard Anodizing but what is Duplex Compression means?

This is a a large machining components with diameter 290mm.

Thanks