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ROI means Return On Investment.

Thank you for the comment and question @cherylsonty. ROI is Return On Investment. If the vote gives you more than what you paid, then your ROI is positive. If it gives you less, then it is negative. It is more often negative, but it has to be reasonable, the goal is to get exposure for the post, not a profit or a huge loss.

I understand better now, thank you very much for the clarification.