DDMC is correct. The NYSE has what is called billable and non-billable. Billable is when you hold a limit posted on the specialist book for 5 min or longer. If you do this the NYSE charges the additional .009 per share. We have built a nice feature to help you keep your costs lower that if you send your order on our system as "Day-NB" then we will replace the order before the 5 min time is up therefore not paying the .009. If for any reason you wish to keep the order live without replacing it within the time intervals then MBT passes through the specialist fee to you. Hope this helps.
Regards,
Steve