What is the purpose of Effective Accounts? What are the three ways to set them up?

Effective accounts: used when the logged in user needs to be able to order for different locations. Can be set up as account hierarchy, see all in an account group, or through use of sharing rules. Settings to also select the parent account even after selecting any of the children. Also settings to have carts per location. Effective account selector is a config setting and can be in header or a widget.