The REL + 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. Although the order type uses the term "Relative," this refers to the behaviour of the limit order initially submitted, which is pegged to the bid for a buy and to the ask for a sell similiar to a Relative order, in an attempt to add rather than remove liquidity. 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 Relative + Market order
When you select REL + MKT, the limit price field becomes inactive and the Trigger Price field requires the combo price input.