Plus

In Pokémon ChampionsGen III

Game

Pokémon Champions

If an active ally has this Ability or the Minus Ability, this Pokémon's Sp. Atk is 1.5×.

How it works in Champions

If an active ally has this Ability or the Minus Ability, this Pokémon's Special Attack is multiplied by 1.5.

“Boosts the Sp. Atk stat of the Pokémon if an ally with the Plus or Minus Ability is also in battle.”
In-game text, Scarlet & Violet

How Plus changed from game to game, newest first.

Gen V
If an active ally has added: this Ability or the Minus Ability, this Pokémon's Special …
Gen IV
If an active ~~dropped: Pokémon~~ added: ally has the Minus Ability, this Pokémon's …

Descriptions: Pokémon Showdown’s for each generation, and PokéAPI’s notes on the changes they don’t show. Champions: Pokémon Showdown.