Validation

Mandatory for what? To submit data? You can include a script in the submit button, or whatever other action this field is required for, that tests for the field's value.

There is also a Mandatory "null" test property for fields in LiveCycle forms. If it is set to "error", Acrobat will display a popup message set by the "mandatoryMessage" property of the field.

you could have check box code that looks like this
if(..check on...)
{
myField.presence = "visible";
myField.mandatory = "error";
myField.mandatoryMessage = "Must Fill"
}
else
{
myField.presence = "invisible";
myField.mandatory = "disabled";
}