The only difference between Limited and Open is that in Open you can use a compensator, optic, and 170mm magazines (28 rds in 9mm) if you wish. Other than what I've mentioned, you're allowed to do more with mag and holster placement. Production classes (and IDPA ESP) limit you to 10 rounds in the mag but you can use up to a 140mm mag in Limited. Power factor is the shooter's choice.