The LMT + MKT order type is available for certain multi-leg combinations, and when used with a non-guaranteed combination order it could help to increase the chances of all legs in the order being filled. Initially one or more legs are submitted as limit orders, but if the first leg fills or partially fills, the remaining legs are resubmitted as market orders.
To create a combination Limit + Market order
When you select LMT + MKT, the limit price field becomes inactive and the Trigger Price field requires the combo price input.