The play is scored as a fielder's choice whether the attempted play is successful or not, so not all fielder's choices result in an out. An unsuccessful attempt to put out a preceeding runner is only scored a fielder's choice if the official scorer thinks that the fielder had a chance to retire the batter; if he tried for a preceeding runner because he couldn't get the batter, the play is scored a hit.
In scoring, FC is also used to account for various other ways in which a runner can advance. The most common use is when a runner advances an extra base on a throw from the outfield, e.g. when a batter who has just doubled advances to third on an attempt to retire the runner from first at the plate. It is also used to account for bases gained on fielder's indifference.