How do I determine when to use a bishop defensively vs offensively

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Zoomer428


Hello friends. I have a chess book that I've been reading and this specifically refers to the colle system..Almost always I choose to place my bishop on the c3 square as it targets the short castle and is pretty well protected by pawns and it prepares my e pawn push. However in the book it specifies e2 for the bishop defensively (for black). How does the person determine this?