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Disparate treatment is a rule which applies to employment actions in general.
If there was disparate treatment which was based on illegal discrimination because of age, sex, color, or nationality, then the disparate treatment is what is used to quantify the discription.
In other words because a person is 65, the person was not given any promotions. This discrimination is the fact that the person was 65, the disparate treatment is the refusal to give the 65 year old any promotion.
I'm switching over to Q&A since you haven't responded.