There is one approval line for all proposals. If they are above the line they get funded, below the line they don't. "Return" is a voter-defined approval line. So, in effect, if a proposal gets more votes than "return" it is approved and if it gets fewer votes than "return" it is not approved. The line can change so if stakeholders think non-worthy proposals are making the cut they can vote for "return" raising the threshold.

EDIT: I guess from what blocktrades wrote it is possible to have more than one "return" proposal (each returning only a portion of the budget) so I guess it is possible have what amounts to different vote thresholds for different portions of the budget.