Question

Name and explain the model used when the user requirement are not clear

Answer 1

    When the user requirements are very clear we could directly start making the product. The development work starts right away. It is called the Waterfall model of development.

    However, when the user's requirements are not known or the user does not know very well what he/she wants, then we need to follow a prototyping model of development.

    We start with a low scaled prototype and show it to the customer. Then depending on the real feedback of the customer, we develop the actual version of the product.  
   
    This saves money, effort , waste of work and resources. It may take longer. But is is better this way.

Answer 2

please provide some basic for understanding your question