Fixed Level of Detail (LOD) calculations are based on the specific dimensions outlined by the user, without referencing what is in the view.
Here is an example of LODs being used in a calculation:
If I'm using Superstore data and I want to display the profit for each region then I can use a fixed LOD. I have called my LOD 'Profit by region'
I can now check if my LOD is working the way I want it to. If I create a table with the Region field and the more granular State field, my Profit by region field should remain the same for all Regions, irrespective of what is in the view. Here, you can see that the Profit by region is unchanged.
This helps us to create fixed variables that we can use to create charts in Tableau!