Some Bikes Like Brompton, A folding bike, made in only 1 size Use the saddle clip as part of the bike fit.
near/far reach.. the setback from the BB-axis changes too .. effectively a seat post angle, steeper or shallower
depending on which side of the seatpost the saddle clip is fitted.
Some seat posts which have been designed to be set back, the integrated rail clip design,
dont have the angle range of adjustment to be in front,
other seatposts, specially made for Triathletes needs, are not designed to be rear set.
Zero setback seat-posts are neither.. clamp mech is directly on top.