In Agile, we start with the top-level idea and explore scope based on what is known related to that idea. In this way, we start with high-level requirements and scope of the product is regularly refined, and prioritized. Since we need to meet time to market, the cost and schedule are made fixed so that in this fixed time and cost, all are focused on developing and delivering to what is really matter.Plz confirm this point :
In agile, time and cost are fixed, the scope is defined per itration
But this may not be true always, many times, cost also varies from the initial estimates if we need to deliver something which is really needed and if it cannot be delivered in the identified cost.
The basic idea is , deliver a product with features which are really needed and when time and cost is fixed team focus on what is really needed and they negotiate scope accordingly.