Basically the idea is to ensure
That youapply from the country where you are living.
If I am holding only an Indian ppt (and working n living in India), I am supposed to apply for Visa (USA/UK/Schengen.....) in India to their consulate/embassy.
If you are living n working - say in USA - as an NRI or on work permit - then you can apply for a 3rd country Visa - in USA.
But, if you are in USA on a tourist visa, no work permit in USA, and you hold an Indian ppt, you can't apply from USA.
Hope there is no confusion on this.