Thanks for the question, @camillagm.
Objectives are set at the line item-level. You can only retrieve the objective information by making a request to one of the line items endpoints—GET accounts/:account_id/line_items or GET accounts/:account_id/line_items/:line_item_id. You are correct to assume that the objectives for different line items under a single campaign will be the same.
Line items with different objectives are not allowed under the same campaign.
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