This filter is specific to reserved instance purchases.
Within the Health Check Report, the Fully Optimized Purchase section displays the highest possible savings (e.g., $85,774) that will be achieved assuming you make the upfront reservation purchase (e.g., $510,065).
The platform scans your infrastructure to determine your on-demand EC2 usage and then recommends the optimal up-front reservation purchase to cover that usage. But because of budgeting constraints, not all businesses can make the fully optimized upfront payment.
The platform uses the setting in the % Fully Optimized filter to calculate savings with a budget that allows for the same percentage of the optimal purchase price.