For synthetic spreads that have yield legs, this field displays the gross basis values for each leg (rounded up two digits after decimal) separated by a colon. In this case, the field acts like a button. When you click the button on a trading application that is using a QFormula, the Formula Editor opens with that QFormula selected. Otherwise, the Formula Builder is opened for you to create a QFormula. When you hover the mouse cursor over the button, a tooltip is displayed.
For a yield contract or leg, this field displays the gross
basis value for that contract or leg: 
To display the button, go to Trading Preferences > Display > Other Functions.