Mark,
In scoping, you can select whether you want the buying center to be based on the account or based on the contact relationship. By default, the buying center facet is based on the account. The selection can be made in business configuration - scoping of account and activity management.
When the buying center is based on the account, the data can be maintained in the account but it is not maintained elsewhere. When you select to enable the buying center based on relationships, this impacts the relationships tab of the contact. You can configure relationship types which are buying center relevant and based on this, you can also maintain additional data points in the relationship tab of the contact. The buying center relationships with employees is intended to show the relationship the employee shares with the contact and how that relationship can influence more selling.