This Substance File's setup is primarily for you to create Tattoos / decals with ease.

For this reason, the best practice is for your edit to be exported alone, without the body texture. 
Ideally it should also have a transparent background.

You should use your exported texture as a Decal within a Unity material. 
Doing so will allow you to avoid atlasing the texture, which you would have to do if you wish to maintain
her ability to have a flat chest.

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If you are trying to edit the base texture directly, you will have to do additional work in Photoshop
to maintain the flat chest option.

Please reference the existing base textures in Unity to understand how the texture must be set up to
function properly.

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Please feel free to reach out in the Discord (https://discord.gg/FmACqk4Y6D) with any questions,
I would be happy to assist.

Thank you!