What is the difference between RELATED and RELATEDTABLE?

The RELATED function in Power BI/DAX is a function that retrieves a single value from a table on the “one” side of a one-to-many relationship. It is often used in calculated columns or measures when it comes to fetching from the related table a value of a particular column for the current row according to the relation defined in your data model.

For example, consider a relationship from table “Sales” to table “Item” (where “Item” is on the “one” side). If every row in a sales table points to a unique item, a column from table “Item,” say Brand, could then be fetched for each row in “Sales” using the RELATED function.