FWIW, some pegged orders such as "REL" allow using dynamic offsets relative to bid/ask to price the order. They are listed among simple order types in the API docs. I haven't ever used them though. I could be wrong but the feature you are referring to is most likely just a user interface convenience of the Order Entry dialog, which is a more hands-on variant of the Order Ticket. It calculates a fixed price immediately and it actually works for any order types rather than being limited to the adaptive algo.
