fields mapped on relations are fields which return a related entity or collection of related entities. Examples:
order.Customer
customer.Orders
order.OrderItems
fields mapped on related fields, are fields mapped on fields of a related (1:1, m:1) entity. This eases the way to expose for example customer.CompanyName through Order, so you can databind a set of order entities to a grid and you'll get the CompanyName as well, which is actually located in a related entity, referenced by the order entity.
