Relational calculus in dbms

I’m solving a problem using tuple relational calculus ( TRC) in DBMS.

Find the name of all the employees who work for XYZ Bank Corporation.

Solution ( as given in my book)

I don’t understand why book is using S . We have t . Since both of them represents tuples , we could just use t in place of S and get moving.

Can anyone please explain why we required S ?

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