difference between fixed model and box model in css

The CSS Box Model
All HTML elements can be considered as boxes. In CSS, the term "box model" is used when talking about design and layout.

The CSS box model is essentially a box that wraps around HTML elements, and it consists of: margins, borders, padding, and the actual content.

The box model allows us to add a border around elements, and to define space between elements.

div {
width: 300px;
padding: 25px;
border: 25px solid navy;
margin: 25px;
}

Fixed Positioning
An element with fixed position is positioned relative to the browser window.

It will not move even if the window is scrolled:
Example

p.pos_fixed {
position: fixed;
top: 30px;
right: 5px;
}