The difference between
Topic position and topic is the difference between a grammatical
position present in all sentences and the word that fills that
position in a particular sentence.
Strictly speaking, Topic is
a position or slot in every sentence, just as Subject is a position
or slot in a sentence.
The Topic position is filled by words that
we, loosely speaking, call "the topic" of the sentence,
just as the subject position is filled by words that we, loosely
speaking, call "the subject" of the sentence.
sentence, the subject position is filled by the words, "the
subject position." Though we call those words the subject
of the sentence, the subject is strictly speaking the slot that
those words happened to fill in that sentence.