Protosynthesis

If Sun is active or this Pokémon uses a held Booster Energy, this Pokémon's highest stat is multiplied by 1.3, or by 1.5 if the highest stat is Speed. Stat stage changes are considered at the time this Ability activates. If multiple stats are tied, Attack, Defense, Special Attack, Special Defense, and Speed are prioritized in that order. If this effect was started by Sun, a held Booster Energy will not activate and the effect ends when Sun is no longer active. If this effect was started by a held Booster Energy, it ends when this Pokémon is no longer active.

“Boosts the Pokémon's most proficient stat in harsh sunlight or if the Pokémon is holding Booster Energy.”

In-game text, Scarlet & Violet