A Stat Modifier can either be a constant value or multiplicative.
Constant values add (or subtract if negative) to the current stat. For example, adding -3 to vitality will decrease the vitality in 3 points.
Multiplicative values increase the values by a percentage. For example, using a multiplicative value of -20% will increase a stat of 200 to 240.
Saving & Loading
This is a decision by design and has a logical explanation. When a Stat Modifier is added it’s usually because of an external event to the character. For example, equipping a sword that increases the strength by 5 or getting a Status Effect that decreases the agility by 20%.
When a game is loaded, all previously saved Status Effects are also initialized, executing their On Start Actions. Same happens with equipment.
If Stat Modifiers were to be also saved, they would be duplicated every time a game was loaded, due to the loading and initialization of their originators and the creation of the saved ones.