Proport Monthly Averages

I am trying to understand Monthly averages (P1, P2....P12) which demantra calculates. Implementation guide says there two ways to calculate monthly averages,

1.) Before level adjustment (it uses system parameter delta and glob prop )

2) After level adjustment (this uses glob prop).

I want to understand what are business scenarios where both these are used respectively.
If we maintain 2-3 years of history then in this case monthly averages will always use after level adjustment method...?

Thanks