Optional Arguments in WHERE Clause

In the below SQL Query, we can see that how only not null arguments will be included in the Where Clause.

DECLARE @SOURCESYSTEM VARCHAR(50)=NULL;

DECLARE @ORDERID VARCHAR(50)='O1001';

SELECT * FROM TRANSACTIONS

WHERE

(@SOURCESYSTEM IS NULL OR SOURCESYSTEM = @SOURCESYSTEM) AND 
(@ORDERID IS NULL OR ORDERID = @ORDERID)