The RELATED function returns a single value from another table related to the current value in the current table. Note: If you are familiar with relational databases, you can think of lookups in Power Pivot as similar to a nested subselect statement in Transact-SQL. DAX can also retrieve a table of records that are related to the current record. In contrast to Excel lookup functions such as VLOOKUP, which are based on arrays, or LOOKUP, which gets the first of multiple matching values, DAX follows existing relationships among tables joined by keys to get the single related value that matches exactly. For example, you can easily retrieve all the sales tied to a particular region, store, or salesperson. It is also useful when there are multiple rows in another table related to the current row or current value.
The ability to lookup matching or related data from another table is particularly useful in situations where the current table has only an identifier of some kind, but the data you need (such as product price, name, or other detailed values) is stored in a related table. Note: Depending on the type of lookup operation or lookup formula you want to use, you might need to create a relationship between the tables first.