How can sentential calculus be considered an independent theory not part of logic?

Tarski says on p. 137 of "Introduction to Logic",
"An example [of a consistent and complete deductive theory] is given by
sentential calculus, which has been discussed in Chapter II, provided
that it is considered as an independent theory and not as a part of
logic (however, in applying the term "complete" to this theory, it is
to be used in a slightly modified meaning)."

What does he mean by "independent theory and not as a part of logic"?
And what does he mean by "complete" having to be used in a slightly
modified meaning?

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