We need to be careful about Rebar placement in construction site. We need to know which direction bar will be at bottom & which direction bar will be on top at the same layer of reinforcement.

Placement of rebar depends on tension/bending along the direction. For example, in Spread footing more tension developed in long direction than the short. So long rebar will be placed nearest to the tension face. Then on top of long bar surface short bar will be placed.

For the same reason in one way slab short rebar placed at the bottom, because of one way load distribution along short direction that creates tension accordingly.

In Retaining wall foundation short direction bar will be placed at bottom because of its cantilever arm feels governing tension/bending at short direction.

In more simple term we can keep in mind that, along which direction the rebar requires more as per the design that bar along that direction will be closer to tension face. Because governing direction bars are required more because of more bending moment developed along that direction.