At dashboard level, it will be not be effective to use the following expression:
=If (Department = 'Sales', ....)
=If (Department = 0, ....)
What he suggested is to break up this different departments into different table structures as this will reduce the IF expression in the dashboard level as we are manipulating the data at script level.
definitely you will end up with snowflake schema which is not really recommended, you can break them by columns instead of different tables, but first why you need to distinguish between the departments? do you have different calculation in your expression based on the department? because at then end if you break them by small tables, columns etc... you will still need to treat them differently in your expression.
again if that is your case you check look for the pick function to solve the nested if conditions.