All relationships in Power Pivot are of the type 1 to many. Imagine you had a table of sales data in Excel, and the table of sales data had product code and qty. But you wanted to know product name and quantity. You could write a VLOOKUP to go to another table, a table with product code and product name to fetch the product name. But what would happen if you table had 2 different names for the same product code? everything would fall apart. When you write a vlookup, you must point to a table that has a primary key (eg product code) and each entry in your sales table (product key) must match only one record in your product table. Otherwise which value will be returned? This is how it also works in Power Pivot