I am learning the concept of complete sentence. An ELL post (Is "It is" a complete sentence?) says
A complete sentence must contain at least one main clause that contains an independent subject and a finite verb and has a complete meaning.
which uses a term independent subject, what does that mean?
I searched on ELL and got only one hit, namely, that post itself.
I also searched on google, no luck.
So, what is an independent subject?
Note: I am aware of the concept of subject, I just don't know the term independent subject.