There are two settings relating to lead cards. One setting is a choice between making lead card entry compulsory or optional. The second setting determines whether the lead card will be validated , if it is entered. If you make the lead card entry optional then players can skip over that field on the contract entry screen by tapping the OK button when the cursor is on the empty lead card field.
We decided to do it this way rather than the first setting being a choice between making the lead card entry compulsory or preventing it being entered. There are advantages in having the lead card entered and validated, for example it can detect that a hand may have been misboarded or the contract entered in the wrong direction.