What is Business Manager (now branded Meta Business Suite / Business Portfolio)? — Meta Ads Glossary — details
Business Manager is the administrative hub that holds ad accounts, Pages, catalogs, pixels, and the people or partner businesses given access to them. It exists so that ad accounts and assets are owned by a business rather than an individual personal profile, which matters enormously for agencies since it allows shared, permissioned access without handing over anyone's personal Facebook login.
Every agency ad account a client uses sits inside a Business Manager, and the health of that Business Manager — its verification status, trust score, and history of policy violations — directly affects how many ad accounts it can hold and how much scrutiny new campaigns receive.

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Private Meta infrastructure for buyers spending $100,000+ a month. Unlimited accounts, our cards, a 24/7 operator line.