RELATED refers to the column RELATEDTABLE() is used to reference a table on the many side of the relationship.
A simple example is to add a new calculated column to count how many sales
there have been for each product.
= COUNTROWS(RELATEDTABLE(SalesFact))
You know RELEATEDTABLE() RETURNS A TABLE, and COUNTROWS() counts the rows in that table. This calculated column in the products table honor the row context