$rowsource(rowsource,column,key1,expr1,…,keyN,exprN) returns the value of the column in the rowsource for the defined rowsource key. The rowsource key is defined as key-expr pairs.
The following example returns the customer group whose ID is given by the value of ${customer id}. In this example, the Customers rowsource has only one key column, ID.
$rowsource(Customers,group,id,${customer id})
The following example returns the standard cost of the part whose part number and location is given by the values of ${part} and ${location}. In this example, part and location are the key columns for the Parts rowsource.
$rowsource(Parts,stdCost,part,${part},location,${location})