Where you choose the "Use best scores" option for the outright report this screen appears. The number of best scores you can choose must be less than the total number of sessions in the event. The other options here allow you to vary the report to suit your needs. The % Field strength adjusted only applies where you have player ratings installed.
This is purely a reporting option and does not change the internal outright scores.
The option [Results][Best scores] does something similar to this option but totals the outright scores and shows them in the [Results][Session scores]. The outright scores produce this way are used for masterpointing, carried forward scores and any related issues.