<part> defines a parameterized component that can be reused in multiple places in the design of a 3D model, and each instance can have a different parameter setting.

Using <part> in the code is highly encouraged because it greatly improves the reusability, readability, and maintainability of your code.

Suppose we've designed a simple frame by cutting a cube out of a larger cube like below.

We can wrap these tags with a <part> tag and give it a meaningful name by adding name="frame" as an attribute. This allows us to create multiple instances of the frame later by writing <frame/>. In the example below, we create two frames and insert a cube between them.

Suppose we want to update the design of the frame part. Instead of a square hole, we want the hole to be a circle. We can simply modify the code inside the <part name="frame"> tag, changing the second <cube> to <cylinder>. Then, both instances of the frame part are updated with the new design.

Furthermore, suppose we want to control the size of the hole of each frame. We can introduce a parameter by adding a <param> tag and giving it diameter as the name. Then, we can assign each instance of the <frame> with a different diameter value to achieve what we want.