it shows as a + play when the outfielder turns a double into a single or a triple into a double.
in the sim outfielders only make errors on dropped fly balls. sometimes they will get to a flyball using their great range then drop it and it shows up as +- in the play by play, but the more common type of errors outfielders make in real baseball (errant throws or bobbling a ball to allow a runner to get an extra base) don't happen in the sim.