Each business has its presentation.
Maintain its name, tax ID, address, logo and branding. The default controls the starting sender on new documents and the business shown in the sidebar.
A sent invoice or estimate keeps its selected business. Changing the default does not reassign that sent document.
The account keeps the work.
Clients, numbering and other account settings are shared. Adding a business does not create separate books, a separate staff directory or a new permission boundary.
Staff access follows clients and money permissions, rather than a selected sender identity. Understand the access rules.