Some phrasing of skill descriptions in-game are ambiguous or confusing. This write-up will help you make the objectively correct and informed decision mathematically whenever needed.
There is a TLDR in Closing Statements and Extensions. It is advisable to skim through all sections and read any that piques your interest more thoroughly.
The first few lines describe the factor, followed by an example. There are some exceptions to this.
Note that, those that are highlighted in Orange are Buffs, and those that are highlighted in Blue are Debuffs.
Buffs are values on your characters you want to increase, Debuffs are values on the target you want to decrease.
Debuffs, with the exception of DEF Stat, can be negative in value.
The word "elemental" refers to all 4 elements in-game, excluding Physical.
Buff factors typically follow this formula: (1 + Buff) while Debuff factors typically follow this formula: (1 - Debuff). More on this in Physical DMG Full Formula and Elemental DMG Full Formula.
The terms “Buffs / Debuffs” refer to only the category itself (e.g. Extra DMG Bonus, Extra DMG Reduction). The terms “Buff factors / Debuff factors” refer to the formulae that contain them (e.g. (1 + Extra DMG Bonus), (1 - Extra DMG Reduction)). They’re different!
Convert all percentages to decimals before substituting into the formulae. (e.g. 20% = 0.2)