Ionic 2 build script platform target

Hi everyone.

I want to customize my ionic-app-scripts, especially here, it’s build platform script.
When i have target ‘ios’ : ionic build ios, then it will compile some scss file I want for iOS.

And I found out that in build.js doesn’t have any information about platform target.
I don’t know where they define platform target.
When i tried: ion build ioo, then that result log was Error: Unknown platforms: ioo

So, where they define platform target ? Anyone help me ?

Thank you so much !
Sr, my english is not good.