Here's everything you need to know about Captains in MLB The Show 24 Diamond Dynasty.
Captain cards are a special Diamond Dynasty series of Cards that can boost the attributes of other players on your squad. In order to activate these boosts, build your squad under certain restrictions.
All cards with Captain abilities are in the Captain card series. Both pitchers and hitters can be Captains, and your squad can have up to one hitting and up to one pitching Captain ability equipped at any time. You can have any number of Captain series cards on your squad, but only one hitter and one pitcher will have an active ability.
First, add a Captain series card to your squad. Then, scroll left from the squad screen to the Captains menu where you can assign your Captain by scrolling up and down to view each Captain series card on your squad. On the Captains menu, you can see how many applicable cards you'll need to add to your squad to activate each tier of your Captain's abilities. You can also press A (for Xbox) or X (for PlayStation) on the squad screen and activate the "Boosted by Captain" filter to see all the cards that work with your Captain.
Each Captain ability will have different criteria to activate based on what type of card you'll need to put on your squad, and how many of those cards you'll need. The harder it is to build a squad with these requirements, the more powerful a Captain boost will be. Captains also always affect themselves with their boosts.
Your Hitting Captain must be in your starting lineup to provide a boost. For the first two-thirds of a game (2 innings in a 3-inning game, 6 innings in a 9-inning game), your Captain cannot be substituted out. After that, they can be substituted, and your team will retain the boost. Your Pitching Captain just has to be on your squad to provide your team attribute bonuses, they do not need to be the active pitcher.