You dont need to worry much about Intra and inter. It is just for classification feasibility.
Intra- Class:
Let’s say your conversion is 1Kg = 2 Pcs. You should do it in intra class. because both the KG and Pcs belong to same unit class i.e. “Quantity”. Go to units form and see the “Classification field group” to find the “Unit class”. ( if you try to define this in Inter class, system wont allow you)
Inter class:
Next, you want to put the conversion as 1 Kg = 1.25 Ltrs, you can do it in intra as both belong to two different classes. So, do it in Inter.