Whats the intuition behind incurable deterioration formula?

Whats the intuition behind incurable deterioration formula? Im struggling to get this formula in my head, simply as it is!

= (effective age / total economic life) * (replacement cost + developer’s profit - curable deterioration)

This is like depreciation.

replacement cost + developer’s profit - curable deterioration is the value of the asset let’s say 100$

effective age / total economic life is how old the asset is. Let’s say it has an economic life of 5 years and its effective age (how old it is) is 1 year so you’ll get a depreciation of 20% (1 year/5 year) of the asset value.

Deterioration = 20% * 100$ = 20$

Whoa, what is this? CFAI page reference please?