Hi @mwasimakram786, I hope you are doing well.
The difference between them is that when you define the variable, you can set a default value you can work with and create your geometry, whereas the set variable value becomes important once you have already defined a variable that needs to be changed to build geometry if something changes.
As an example, you can define the thickness of a lane layer as the sum of two input parameters through a variable. However, under certain circumstances of cut or fill height, you might prefer layer thickness to have a different value. For such a task, you would assign the variable (inputparameter1+input parameter2) to the layer thickness. Then, you would evaluate a Decision for testing cut and fill height, so if the value exceeds a defined range, you would use the set variable value to make the layer thickness bigger or smaller.
I hope this helps. Best regards!
Camilo Fernández
Civil engineer | Specialist in design, construction, and maintenance of roadways
