When identifying target records that match the source record, a statement is created with this format:
Where <target field> = <source expression>
The source expression in the "where" clause represents one or more source values that can be calculated or reformatted using the Edit Formulas dialog box.
Numerous operators are available. You can optionally apply a scalar SQL function to the target and/or source fields for "fuzzy" matching.
See also
Using Operators in a Lookup Expression
Using Scalar Functions in Lookup Links