Actually I want to know what does it mean by alternative uses of this two terms of economy.
Scarce resource : is a (physical) limitation which forces us to make choices accordingly. It is what is available under current technology and cost of extaction. We could say its a supply side perspective and generally refers to material.
Unilimited desires: Is a demand side perspective. Individuals or organizations usually want more tomorrow than today. Even if you get satiated of one good you will want more. For instance if you have enough foood you can't eat more but maybe you would wish a special meal in a fancy restaurant. So even if cooking home was sufficient to narch your need, after an increase of income you will buy more services... bigger house... There are diminishing returns to consume more of the same good but imagination makes it possible to always find a new object of desire. Thus unlimited wants.
By combining these two phenomenons, a scarcity situation arises and humanity must make choices hopefully to fullfill the most pressing needs.