Business ID’s refer to your Business Manager (BM) account, which is essentially a container for all of your various Facebook assets, including: Ad Accounts, Pages, Pixels, Apps, etc. Ad Accounts are used to create campaigns and ads, while Business Manager is used to create Ad Accounts. Pixels can also be created from BM. BM is primary used by teams to manage admin permissions across multiple assets, from one place.
With a BM, you are able to create up to 100 pixels. With an ad account, only 1 is possible. So if you are looking to have multiple pixels set up on your website, you’ll need to create them at the BM level.