On the left is a non-parametric rectangle and on the right is a parametric symbol.
Parametric symbols can be any combination of numbers and it instantly redraws itself.
The Symbol First create the symbol drawing with dimensions as illustrated in the Ashlar manual. The file should only contain the symbol and its dimension lines and no other geometry or text. Open your other, work file into which you will be bringing the symbol. If we had a more complex symbol all the Independent variables would show in the input window. That's all there is to it.