A business file (or a file) is a logical grouping of services that interconnect, share data and have to be synchronized together.
For example, if your business/company is using 3 different services (and connectors) you would group them in a single Business File.
Contacts across all of these will be synchronized and de-duplicated together, and transactions would flow from one service to another.
OneSaas is charging per business file per month.
No data is ever shared across multiple business files.