Just Notify
Changes nothing. It only makes the target Stat update whenever the Injector Stat changes.
Use it when the target's value depends on another Stat in a way the target computes itself — a State Based Effect, a Conditional Modifier or another Linker that reads a third Stat — and that Stat's changes would otherwise go unnoticed until the target updates for another reason.
Example
A State Based Effect LowHealthRage is active while Health ≤ 30 %, but its condition reads Health inside the Stat itself. Add a Just Notify Linker to LowHealthRage with Injector Stat Health: every Health change now updates LowHealthRage, so the state switches the moment Health crosses 30 %.