TradeStation Help
You can use this option to reflect a Bid/Ask at a lower/higher price than you are willing to pay using a specified price increment.
For example, stock ABCD is 10.15 Bid and offered at 10.35. You are willing to pay 10.25. However, if you bid 10.25, the seller, thinking that there is an interested buyer, may move higher to 10.50. By joining the Bid at 10.15 with a discretionary price of 10.25, the seller, believing that there are no interested buyers, may drop his offer to 10.25, which would result in your trade being filled.
This Advanced setting is available only with specific Equities order routes.