In NotifyMe, every person belongs to a company. These may be either business units (if they are internal to your organization), or business partner (in they are external).
Business units are companies internal to your organization, and are used to group users.
Depending on your organization's structure, you may want to create business units within your organization in order to group users that work together closely in the same business unit (e.g. by geographical location, or type of business).
Business partners are companies external to your organization, and are used to group guests.
Business partners can be for instance customers, suppliers, or inspection bodies, depending on the type of relationship that they have with your company and its supply chain.