Where is the system using what I type in the field 'Business EIN Number' field under My Account > Coach Profile?

The system will add your business number in the body of the invoices that you send to your clients.  Some coaches choose to include their number here (and it is then automatically displayed in the invoices for all of their clients. Other members choose to not enter their number here, but in the "Add a Note" section when creating an Invoice (because the coach can then pick and choose which clients need to receive this number).