Models aim to gain a better understanding of an aspect or feature of the world by making it easier to visualize, define, quantify and the like by referencing it to something more commonly accepted by all. There are different types of models and each can be used depending on the aim.
If you want people to better quantify a certain aspect, then mathematical models can be used. If you want people to better visualize a certain aspect of the world, then graphical models be best. These are just some examples of models.
Explanation:Selectively permeable means a membrane allows the passage of some molecules or ions and inhibits the passage of others. The capacity to filter molecular transport in this manner is called selective permeability